Problem appeared very suddenly today, and rebooting didn't help. I tried multiple tracks in my music collection in both Rocket Player (my preferred app) and Music, and got the same results. Ringtones sounded fine played via settings, so I think I can rule out hardware.
Right now, I'm wiping out the iSyncr directory and will re-sync from iTunes. If that doesn't solve it, what next?
Right now, I'm wiping out the iSyncr directory and will re-sync from iTunes. If that doesn't solve it, what next?