Ok, so it's not the obvious thing. Are you running any plugin effects, particularly on the main mix output bus (A1) which could effect it? Try going to AUDIO PREFERENCES and hitting the box for RESET REALTIME EFFECTS to see if that makes any difference. If there's still a problem and Media Player is working ok, it's possible a line of code has got corrupted and it's probably worth un-installing and re-loading Power Tracks.

ROG.