Try the first half of this FAQ for both MPEG and AVI files.
I had exactly the same problem on one machine with MPEG files and doing that fixed it. I don't know why but it worked. Then I discovered MP3 files no longer played in Opus and did the same thing and they started working again as well... I figure something breaks some registry settings and temporarily restoring the associations to Windows Media Player fixes things
For WAV files, Opus doesn't play them using the preview pane by default. If you double-click them (and have set the WAV option under Preferences / Double-Click / Files) then they'll play in Opus's little sound player window (which you can also access with the Play command), but the viewer pane only shows them as hex by default.
All is not lost, however. If you go to Preferences / Plugins / Viewers and configure the ActiveX Document plugin then you can add .WAV to its list of handled extensions and it will play WAV files in the viewer pane via Windows Media Player.