Okay, issue has been reported already. Just gotta press Return after entering new values. Still pretty unintuitive! See http://www.wildfiregames.com/forum/index.php?showtopic=19391
Hi everyone, just noticed that once I edit some of the music volume text fields like 'music amplification' from 0.2 to 0.9, this change will not be kept after reopening the options window. There furthermore is no audible change happening as well. Love 0ad though, great game so far!