some xvid avi cannot be played by media player...

I bought a media player which you can connect hardisk on with u***** connection.... and the player directly connect with TV (audio and visual)

it can suppose can play xvid and rmvb...

but I fould 2 problem:

1.not all the xvid avi it can play, why? because too big file or there are a lot of different xvid format?

2.can ply rm file, but no sound... why?

I like xvid file, because it is smaller in size, but with good quality.. but if it cannot be played... so what should I watch when I try to convert other format to xvid avi???

thanks for viewing!