having exactly the same problem on my N900 too: media player does not play anything any more

tried all suggestions mentioned in this thread without any success.

i also have panucci and mediabox installed, and these seem to work just fine (i installed mediabox after built-in media player stopped working)

what i have noticed:
in mediabox there is a setting "Select Media Renderer" (under the status bar menu, _not_ in the app's 'shelf' settings section), and when this is set to "Local" (the only option there) then even mediabox stops playing anything (until the app is restarted)!
maybe this can give one of the gurus here an idea about where to look for a fix...

gonna give it a couple more days before reflashing in case someone comes up with another solution...

Azadi