Will wiping the cache partition have any effect on "cached" Google Music albums?
Sent from my DROID RAZR using Android Central Forums
From what I could tell, offline Google Music files are stored on either your external SD card or the phone's internal memory (also viewed by the system as an SD card). According to this article
AddictiveTips ? Blog ArchiveAndroid Partitions Explained: boot, system, recovery, data, cache & misc
the system cache partition is separate from the SD cards, so my guess is that wiping the system cache partition will not affect any offline Google Music files you may have downloaded. But if you end up losing those files anyway, you could always download them again, since you still have them in the cloud, right?