I'm having similar problems, I had the sound drop out exactly as Nick described prior to upgrading to v2.5. After upgrading this appears to have gone away to be replaced with (infrequent) complete failure to lock (indicated by NL on the display and wild oscillations in the front panel indicators). Only a hard reset fixes this. Since the upgrade I've also noticed small "clicks" with track and volume changes, the latter very obvious when adjusting volume at high levels (>70 or so) on DVD-A. This is with an upgraded 568.1 and 598D via smartlink.