Try to install package MPlayer-Plugin; its deb is intended for Maemo 4 Diablo, but since its armel, too, it should be workable. I have installed on Maemo 5 Fremantle the GoldenDict package intended for Maemo 4 Diablo - and it works quite well.
After installation, this plugin should be listed under MicroB->Options->Add-ons->Plugins
Currently, I observe Atlas Plugin, Default plug-in, Mediaplayer media plug-in and Nokia MessageBus Plugin in this list. You should see MPlayer plugin added to the list.
Afterwards, if there are problems with persuading MicroB to open media files with MPlayer plugin, ask. I have recently persuaded Firefox to open avi video files with Windows Media Player Firefox Plugin.
Best wishes.