Audio stream... problem

[spaeter]

Hallo.

this is my first message... I hope this is the right section and that you can help me

I've a little problem.
playing an avi file, i don't have access to the audio stream option (right mouse, audio...)
the functions dsp plugin, audio streams and mpeg audio channel are not activate (are both in grey)..

how can I activate these functions?
for example to change the ac3 setting playing a film?

thanks and... sorry for my english...
bye!!