I can't play .ogg files with Iron 5 but yes with Iron 4.
I have tested other formats (wav,mp3,m4a,aif) and plays with no problems.
I have VLC 1.05 installed with a clean install and with all plugins.
"about:plugins" shows the plugin "npvlc.dll" (from VLC) and there's the mime type for ogg files in.
I have tested Firefox and GoogleChrome 5, so, to see if the problem would come from Chromium 5 but it plays correctly. I suppose the problem comes to the new version of Iron.