There is a rare bug in ZK where the music occasionally plays slower than normal. It also has a noticeably lower pitch. I heard about a 'feature' which changes sound speed based on game speed. This might be a culprit and I would like to turn it off. There may also be a bug with Spring.PlaySoundStream and I do not know why the widget uses that callin than Spring.PlaySoundFile.
Do these sounds require an entry in sounds.lua? They have an entry with these settings: