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.