Have no problems with music files. The only thing (so far) that has gone south is the You Tube videos. Seems that ICS does not support Flash. Your videos are okay, it's only the You Tube videos.
I've been on the update (ICS) since early Saturday morning. I don't have a vast music library. I do have some mp3's (used mostly for ring-tones, alarms, etc.) and they play just fine with the Play Music widget. Youtube is working fine for me as well.
mortb: Flash is supported in I.C.E., in fact they released a new I.C.E. version (11.1?)
It's Jelly Bean that loses Flash.