I usually played zoomed out, so I rarely hear any ambient sounds.  I have heard moving ships if I am zoomed in enough, and I have heard a star occasionally.  I just tested what I could hear in my current game, and it wasn't much.  I heard what seemed to be the same sound coming from just a few yellow stars and the same for purple, blue and orange stars.  I didn't check them all, but most did not have sounds.  I checked all of the white stars, and none had sounds.  I checked planets, shipyards, starbases, precursor relics, nebulae and everything else I thought of, and none had sounds.

I had turned off music, turned SFX volume and my computer's volume to maximum, so I shouldn't have missed anything that was making sound.

Edit:  and I saw that 00zim00 posted while I was writing the above.  He has probably explained why I didn't hear much.