I'm hoping someone will have some answers to this....

I have put some films from my laptop onto my phone, most work beautifully and I'm so impressed with the quality but a couple of them say the audio codec isn't supported when using the generic n900 media player.
Using mediabox the same videos do play but have no sound.

they are all avi files, so seems strange some play and some dont, and the same ones have problems in both media player, all be it different problems!

AVI is just a container. Audio can be encoded in different formats.

That's right!
You can also use kmplayer or mplayer which natively have more codecs on board.

thats brilliant thanks! km player is amazing, didnt realise could use it generally I thought it was just for world tv, have just discovered blip downloads and some turkish language lessons. Partner huffing and puffing as wants to go out and Im marvelling at all the possibilities.
starting to love my n900 more and more as I discover the possibilities thanks to lovely people on this forum!