When I play SA whenever I go to sandbox it plays Nauvis soundtrack even tho I moved on from that planet a while ago. Would've been nice to have at least an option in mod settings to choose a planet from which the music tracks are gonna be played.

I would have loved to do this, I actually had plans to make custom music for the Sandbox. In the FAQ I've addressed this and provided a workaround (for the No Isolation mode), which was accurate for 2.0. I provided feedback on that music system before Space Age launched, because it was only possible to make music like that for Planets, not any old Surface.

I saw some music-related changes in 2.1, and I'm going to go through them in more detail soon. It may be possible now, but I think it required known Surface names in the prototype stage, whereas we only know most of ours at runtime. There may be a compromise in there, though!

Thanks for the FAQ entry, that pretty much solves this feature request in the best way possible. Just for the record, in 2.1 sandbox plays Nauvis' music by default (unlike 2.0 where there was no music at all).