It's one of my favourite. You know, the game is very simple made, but omg this sound!
Also the programmers did a great job using non-standard approach to music. Usually they create a "battle theme", "wander theme" etc. and trigger them by some conditions. And this is different. You almost never hear the whole theme, but rather various mixing of its tracks. The game just shifts a mix ratio according to the progress which adds to atmosphere a lot.
I've unpacked the game files and the soundtrack is indeed is stored as multitrack i.e. each instrument in a single file. But here you can hear it as a whole:
[youtube]
Full soundtrack:
https://bandcamp.antonriehl.com/album/receiver-music-from-the-game