Lipsynch, Barbican, London

Robert Lepage will be called a wunderkind until the day he dies. But as he coasts into his second half-century, "the marvellous boy" has developed into a deeply mature adult. As well as wowing us with the theatrical wizardry that has always been his hallmark, he's now willing to share open-heartedly the fruits of the emotional wisdom he has evidently learnt from first-hand experience of pain, confusion and loneliness.

I won't misuse the term "quantum leap", for with his omnivorous interest in science, Lepage would wince at the illiteracy of the image. But it's true to say that of late he has entered a new and altogether more humane phase of brilliance. We saw this in the recent autobiographical piece, The Far Side of the Moon. It's even more richly evident in the rampant global and inner-world reach of Lipsynch. This latest wonder uncoils over nine hours (succinct, by Lepage's standards) on the Barbican main stage. It's mind-blowing, heart-breaking, hilarious and beautiful beyond words.

Lipsynch is structured in nine units, each of which concentrates on a figure from the floating gallery of characters who populate the overlapping, sometimes concentric, sometimes left-field plots. These various strands are gathered in a climax that is unbearably moving because, though literally operatic, it is so un-milked and dignified.

As the title suggests, the show uses as a metaphor the multifarious aspects of sound technology: dubbing into a foreign language; miming; lip-reading; created multi-layered tracks; doing voice-overs; being the tones associated with a particular product; speech therapy for the neurologically damaged; even (though this is given a black twist that strangles the hilarity) the voice that intones, from permutations of many prerecorded possibilities, the reasons why a British Rail train ("object on the line") has been cancelled.

The situations thrown up by the multilingual plots mirror one another in many revealing and disturbing ways. As a subtitle for Lipsynch, "Nine Characters in Search of Their Lost Voices and of a Way of Getting Back Home" would not be inaccurate. On the deepest layer of the palimpsest, there's a lovely Nicaraguan teenager (Nuria Garcia). She is tricked into leaving her native land by a hip German couple and finds herself trapped in Hamburg as a sex slave. She dies on the flight to liberation, leaving behind a baby boy who winds up being adopted by an opera singer (an intensely affecting Rebecca Blankenship) who was on the same aircraft.

This girl's story is part of a pattern that propagates itself everywhere in the piece. It's one of step-parenthood and of finding – or rather of creating – the truth in the post-production stage of art. For example, there's another singer (Frédérike Bédard) who has lost whole chunks of her memory through a brain tumour. She employs a deaf woman to lip-read what the father who died when she was 13 was saying on certain silent home-made family films. She then hires a voice-over artist to dub his banal words on to the film. But it's only when she uses her own voice – puts the words into the paternal mouth – that she magically summons his lost tones into the room. As with the final image of a gender-reversed pietà – the now adult baby boy cradling the child-woman mother he never knew – there's the Joycean idea here that we have metaphorically to beget our parents anew before we can make a true start in life.

The piece is startling in its many kinds of excellence. You can hardly believe the comic genius of the cast's timing in the section where the son is seen making a film of fictional speculation about his mother's life – lots of egomaniacs tripping over one another's self-absorption. Then there's the music, such as the sequence where the son, seen as a spotlit face in the window of an aircraft, sings Gorecki, unearthly in its weird, soaring loveliness; or the a scene in a Soho jazz cub where Bédard delivers a barkingly bravura version of "April in Paris", a piteous mixture of dread and accusation. Sarah Kemp is magnificent as an incest-wrecked Northern prostitute, and John Cobb is side-splitting in drag as an ancient speech therapist with Alzheimer's. Mandatory viewing.

To 14 September (020-7638 8891)