I am currently running CM11 (Android 4.4.2) on a Galaxy S2, have most of my music on an external SD and I'm also experiencing the freezes since CM11 M4. Thanks to this thread I tried formating the card to NTFS and deactivated all fading. I started Poweramp and just let it play for itself, to see how it's behaving. The issue seemed to have gone, but after a little over 6h it froze again. If it only happens after 6h of constant playing, which I'm not likely to do, it would be fine. But since the issue isn't solved by this workaround evidently, I suspect it will randomly occur after less time, too.