See this FAQ:
[ul][li]Enable/fix playback of various media formats in Opus[/li][/ul]
Also, if you're using 64-bit Windows, be aware that Opus uses the 64-bit video codecs installed on your system, not the 32-bit ones that Windows Media Player uses. (Unless you're using the 64-bit WMP, but that's very rare; the 32-bit one is usually used by default.) If you're on 64-bit Windows then chances are you need to install/update the 64-bit version of one of your codecs. (You can have both 32-bit and 64-bit versions of the same codecs installed.)