I recently dug out the mixes from a soundtrack I wrote back in 1992 for a series of self-hypnosis videos by a blossoming superstar in the field, Paul McKenna.
The soundtrack was based round a didgeridoo, which was a logical choice given that the purpose of the music was to take the subject into trance, form a background for their relaxed state and then bring them out. The other instrumentation was piano, sax, percussion and nylon string guitar.
With the drone of the didgeridoo, I was limited to chords that worked with its single note tonality. The didgi player had two instruments, each “tuned” to a difference note, so some variety among the tracks was possible. It was an interesting challenge that produced surprisingly musical results.
Unfortunately, the recording was rejected by the production company, with one comment being that it sounded “too Australian”. Eventually a synth-based “new age” soundtrack was used, which was less interesting. However, in this case, being less interesting probably better fulfilled the brief.