I (probably incorrectly) thought that MP didn't support ogg, even with the support added. This is from the wiki entry on ogg support - http://wiki.maemo.org/Playing_OGG_files

On re-reading it doesn't say it doesn't, just doesn't say it does.

Anyway, partway through fixing it last night, I accidentally opened MP and not OMP and noticed that it was showing them (in fact identical behaviour). So I guess that the issue was at a lower level - the DSP?