MP3 's don't play in view pane

Sometimes assigning MP3 to WMP (then later back again, after seeing if it worked at all) makes things work again (since it seems to make WMP re-establish the relevant registry settings for the ActiveX control).

Info on how to try that is in this faq: HOW TO: Enable/fix playback of various media formats in Opus

If that doesn't work, I'm not sure why the WMP ActiveX control wouldn't be able to play the files when WMP itself can. You could try turning on the WMP preview handler instead (via Preferences / Viewer / Viewer Plugins and the ActiveX + Preview + Office + Web plugin there); the preview handler is not as good as the ActiveX, though, since you have to manually push play for each file. (Unless you want it that way, of course.)