I noticed it too. It seems to be limited to AAC+ streams, and it sounds like the problem is the SBR decoder not kicking in, so you lose the top half of the audio spectrum. I hope this doesn't make it into the final version, which I don't see why it wouldn't as the decoder itself isn't broken, like in Android. (AAC+ still works perfect through the first party iPod app.) With mobile data caps AAC+ is widely used to keep audio bit rates reasonable and still sound decent, and it's VERY prevalent so this needs to be fixed.
Spotify though? They're using their own Vorbis decoder. I'll have to sub for a month and see if it works OK. I'm not going to continue to pay for MOG while this is broken so its a perfect opportunity to see if spotify really does have problems.
EDIT:... And spotify is broken too. What the hell did apple break here? They're using their own decoding library and its still broken. Especially considering Melodies is working ok for me.