This might or might not be up your alley, but I totally forgot to include some Musique Concrète: Pierre Schaeffer: And speaking of schaeffers, there's also R. Murray Schaeffer, who sometimes writes not just for instrumentalists, but also the environment - allowing nature to interact with the musicians, or placing the musicians around a lake to take advantage of the natural reverb and echoes Steve Reich is another composer you might dig, if you've never heard. I wasn't sure if you were looking for earlier stuff, so I kept to the last 100 years-ish.
The Musique Concrete guys are a little amusical for my tastes, but I appreciate what they do. So I typed "steve Reich" into my iTunes to see what comes up - I have a bunch of ambient weirdness from backintheday that I bought in bulk and I don't always know what it is. One song came up: By damn.The song also uses a harmonica sample from Ennio Morricone's The Man With The Harmonica (from the film Once Upon a Time in the West) and parts of Electric Counterpoint, a piece for multitracked guitars composed by Steve Reich and recorded by Pat Metheny. Reich was "genuinely flattered"[8] by The Orb's use of his work and instructed his record company not to sue.