I'm not sure if it's related to the music player, but since I've listened to music on my SGS i've got battery drain and high cpu usage (800+ MHz) too. So i sat down and tracked it a little bit. Looking at the output of top, i saw that the process system_server was using 75% of the Cpu and more. I've stopped any service if possible, still no luck. So i switched the lockscreen to another one, and now cpu usage is down to normal, even if i switch back to the original one.
Hm its not releated to kernel its a MIUI bug.I replased music.apk with oldest version and for 1h battery drop with 18%.Now i'm on previous release of rom and today for 1h baterry drop with 8%.I tried cMIUI and its the same.I hope tomorow this will be fixed bcz i like MUSIC.