No literally, it’s the sound reel from the film.
For many popular films, the soundtrack, it seems, has been reduced to a crass marketing ploy -- stuffed with filler from big-name acts “inspired” by the movie (or, if you’re lucky, stuffed in to the end credits), a soundtrack becomes a compilation of mostly C-grade material from the top 40. Put See You at Regis Debray, Ryoji Ikeda’s soundtrack to CS Leigh’s film, at the opposite end of this spectrum: over two discs, Debray includes the entire sound reel from the film, including both Ikeda’s soundtrack and moments of only natural sound.
To begin with, your suspicions are correct -- the passages which feature only the sounds of the one actor in Leigh’s dialogue-less movie are generally dull and, while the idea of the inclusion of said sounds is interesting on paper, in practice it proves easily skippable. The other half of the soundtrack belongs entirely to Ikeda, and features some of his most gorgeous and, it must be said, accessible work. Leigh’s film portrays Lars Eidinger as German radical communist Andreas Baader, of the Baader-Meinhof group, and the things he might have done while staying alone at the French apartment of philosopher Regis Debray in 1969; the liner notes explain which sounds accompany which visual action.
Not having seen the film, I can’t say whether Ikeda’s music fits, but the epic minimalist synthesizer swells he’s prone to appear fascinatingly appropriate to accompany one man’s engrossment in the act of “Listening”, while “Tearing” features the crisp sounds of paper being ripped over Ikeda’s bet of reverb-heavy, western-style guitar plucks. “Tearing” is further aided by the progression of the piece to include a bass part and more guitars, which lead into “Polaroiding”, shockingly non-experimental coming from someone like Ikeda. “Masturbating”, of course, appears awkwardly without any musical accompaniment. All in all, it sounds like one hell of a series of emotionally intense, yet physically minute events, which makes Ikeda a perfect sonic counterpart.