'Opera," says composer John Harris, leaning in across his kitchen table, "is the thing.

Of all musical and theatrical forms being created at the moment, opera is the one that all sorts of artists are putting all sorts of creative muscle into. Why? Because they feel they can push at boundaries in opera that they can't elsewhere." Harris sits back in his chair, his wife, playwright Zinnie Harris, watching him quizzically. A drone of jangly CBeebies drifts in from where the couple's two youngest are watching telly in the sitting room next door.

Given popular (mis)conceptions around opera – that it's stiff, that it's stylised, that it's hard to penetrate and, most common of all, that it's elitist – Harris's words deserve a moment of reflection. First off, is opera really attracting artists from outside of the classical establishment? To think of some of the most widely publicised premieres of recent years (Nico Muhly's Two Boys, Damon Albarn's Dr Dee, Rufus Wainwright's Prima Donna), Harris has a point. These might not have been great operas, but they did bring new voices into major houses.

As for the boundary-pushing flexibility of the genre: think of the range of opera shorts we've seen in Scotland alone. A Louise Welsh crime thriller; Craig Armstrong's multimedia Ibsen; a "folk-opera" by Anais Mitchell; a "sort-of opera" by David Shrigley. On surface level, at least, these have looked and sounded worlds apart.

Given that, what does the term "opera" actually mean nowadays? This year's Sound Festival in the north-east of Scotland turns its focus to exactly that question, hosting talks and workshops exploring the multifarious scope of opera today. And, as if to prove the point, the festival presents a series of short operas staged in weird places. There's one on a bus, one in a lighthouse, one in a stables, one inside – inside – a desk.

John and Zinnie Harris have teamed up to write an opera for the kitchen of a tenement flat. "A kitchen-sink drama," John chuckles, except that it's not really. The Garden is a reworking of Zinnie's darkly atmospheric play of the same name, first seen as a breakfast show at The Traverse in 2009.

"It's set in a non-naturalistic world," Zinnie explains, "some time in the future. The woman is on her own at home and the world outside has become very hot. The man is out trying to write a report on the growing population crisis. Their relationship has pretty much disintegrated. Then a plant starts to appear through the kitchen's lino floor. The woman can't work out how it's getting there – the flat is six storeys up – but every time she cuts it back, it reappears." Eventually we realise that the plant is an apple tree, that the couple represent Adam and Eve, that the garden of the title is in fact Eden. "A simple idea, really," she says. "A parable."

The Garden worked well in its original form. Reviews at the time called the play "an astonishingly moving portrait of a loving couple at the end of their tether" and admired the writer's "crisp, elliptical style and sense of political engagement". So why bother turning it into an opera? "Because we felt that there was something particularly intimate about it," says Zinnie. "We knew that the experience of watching it as an opera rather than as a play would make it more intimate still."

And that answer cuts to the heart of why opera, more than any other art form, has the potential to twist, to play with and to generally heighten our emotive responses to drama. "The music takes us into a more emotional place," Zinnie shrugs. "It's a much more involving experience," adds John. "It's impossible to sit back in opera. The music's emotionality runs the whole time."

If music can add layers of emotional complexity to a play, it is conversely The Garden's plot simplicity that makes its transition to opera feasible. "Opera is an odd beast," says John, "because it is terrible at narrative. There's the horrible phenomenon in modern opera where composers try to set mundane actions in real time. Someone sings 'pass me the sugar' and it takes five minutes for the sugar to be passed. Nothing can move quickly in opera because the music takes time to do its thing."

Mundane things do happen in The Garden – the man asks the woman to get a beer from the fridge, for example – "but it's not about whether she gets him a beer out of the fridge," says Zinnie. "The point is that he keeps asking but never listens to her reply. What's important is the emotionality within the gesture. And the music helps to make that distinction."

The Harrises work together fairly regularly. Mostly John provides music for Zinnie's plays, but both agree that opera is by far the more mutually fulfilling mode of collaboration. "In opera there's a musical narrative and a theatrical narrative and they have to match up," says John. "In theatre the music is just plonked in to serve the narrative. Words and music are never equal partners."

"Whereas in opera," says Zinnie, "they're more than equal partners."

Together they make a fiercely, complementarily bright duo. Zinnie studied biology at Oxford, John physics, and as artists their inspirations range far and wide (their previous opera together, a Scottish Opera 5:15 short called Death Of A Scientist, was based on the weapons expert David Kelly). They both work from home but, emphatically, in separate rooms. "John hums all the time," says Zinnie, "which drives me mad. And I mutter while I work, which drives him mad."

She reduced her original script from 30 pages to about 10, which he then set using electronic instrumentation. "It's all old-fashioned synthesisers and strange, low, wobbly sine-tone generators," he says. "Sometimes there's a sample of a buzzing cicada in the background. One way or another there's sound going on all the time – often crackly, ancient electronic sounds. My physics background is popping out all over the score." The vocal score mixes speech and song – "opera is open-form now, so we're able to play with those distinctions" – and the close intervals of the electronic underscore have the effect of making even straight speech sound somehow musical. The text, like the story itself, is lifted into a kind of hyperreality.

With that in mind, the Harrises decided to cast singing actors (Pauline Knowles and Alan McHugh) rather than acting operatic singers. "It's being performed in a small space so we don't need the singers to project massively," says John. "And because we'll be so close to them, they'll need to have very good acting skills to carry the drama. Also, because opera singers produce their singing voices very differently from how they speak, it's often awkward for them to transition between the two. Actors have – dare I say it – a more flexible approach to their voices. They're interested in the gradations between the two."

Considering this approach to casting and a score full of spoken passages, a very basic question creeps to mind. Why call The Garden an opera at all? Why not, for example, call it musical theatre? John draws his distinction along formal lines. "The score is through-scored and the majority is sung. We don't have big show-tunes. In musical theatre, like in conventional plays, there's a narrative structure into which you insert show-tunes. Even something like West Side Story is definitely musical theatre because it follows that framework."

Sound's commissions encouraged composers to "try to stretch the musical concept of opera as far as they could on a limited budget". What's interesting is that even a platform expressly designed to push at boundaries still generates discussions such as these – discussions rooted in the formal conventions of genre. Opera is still stylised, and that's precisely why it's fun to play with. Zinnie sums it up nicely: "For many artists, contemporary opera is appealing because it requires us to engage with tradition in a way that people will recognise and feel comfortable with, but also to move forward within that tradition." As is so often the case, freedom with rules is the greatest freedom of all.

The Garden is performed at a secret venue in Aberdeen (ticket holders will be advised where to meet) as part of Sound Festival on November 2 and 3. www.sound-festival.co.uk