How can I add more music into the game?
Alternatively, how can I extract the game's music? (To add to a larger playlist played in background).
Thanks

The soundtrack is in Stars in Shadow\Lua state\GUI\music, in Ogg Vorbis format. You even have an original_tracks subdirectory containing, well, the original tracks in MP3 format before they were converted to OGG and slightly tweaked to better fit as game music. Any music player that's less than twenty years old ought to play both the OGG and the MP3 files without issue.

Interestingly, you'll find in the original_tracks folder one unused song, "Lunar Expedition".

For adding more songs to the game, you'll need to take a look at the @Music.lua file in that same folder. That files shows a few other unused tracks, commented out, that are not actually found in the game at all, like "Rescue Mission" or "Cyber Consipiracies" [sic]. Mostly it seems to be very technical (start and end times for the loop points, it might make sense to move that to Ogg Vorbis metadata BTW) and adding more songs doesn't seem to be very complex, just a bit tedious to set up as you have to find out the magic values for your new songs.

Thank you!
It did not occur to me to look inside the GUI folder for the music, I'd assumed that only contained visual stuff. (Silly me for assuming)
Looking at the Music.lua file, it does seem easier to just run my audio player in background with whatever playlist I'm feeling at the moment.