Perhaps i'm one of the minority here but in most cases i total mute the game's soundtrack, sure in some cases like DoD back in the day and more recently CoBD i leave the sound on but even on games with voice work in like Pine Falls i just mute them.
For me in games sound is either for ambience or for feedback, if i'm playing dirt rally i want/need to hear the engine, in Xcom or w/e i like it for audio conformation of a hit but it's not necessary, if i'm playing an RPG then it's ambience. What i'm trying to get at is that sound isn't a big deal if you don't make it a big deal, using simple commons free music so that you have some sound is okay, it's not going to make or break your project.
Don't overthink it, soundtracks come into their own when everything else is on pause, think of legend of zelda, great themes/music right? well they come into their own while your walking across large swathes of map not doing much else, similarly a great theme over the credits while your not doing anything, then you can enjoy the music, a game with a lot of reading or button mashing means the player is busy, soundtracks in VN's or "busy" games like shooters are much less important. what i'm trying to say is, is your game the sort of game with lots of mindless tasks?, is it a hiking simulator?, don't need to concentrate on other stuff much? well if so then the soundtrack will make more of a difference, otherwise it's incidental and you don't need to worry about it too much, grab some commons free music that fits your genre/feel for the game and run with it.