edit avi file, need to change audio to ac3 or mp2

You should be able to use "avidemux" to convert the audio part of your file (as well as the video part if you want to).

I have avidemux and tried it but nothing better happened. I am guessing that avidemux needs transcode and there is a problem:
In add/remove programs transcode does not show, in synaptic it does, together with a number of other related things.
When I forced add/remove to do a reload, I received the following error and it did not finish downloading:
W: GPG error: http://packages.medibuntu.org dapper Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2EBC26B60C5A2783
thank you for you help Jigho
Costas

One more thing in synaptic under Third Party Software http://packages.medibuntu.org dapper is included as free and non free and also as source code. The same error as above is shown when reloading from synaptic. I also noticed that when I clicked on the link http://packages.medibuntu.org/ it took for ever to load an index page with three entries:
dists/
medibuntu-key.gpg and
pool/
Maybe http://packages.medibuntu.org has a problem today or needs a special verification. Earlier when I tried again it did not load the page and showed me an error that verification is required.
Maybe the same problem causes the program remind (alarm, calendar etc) to show as installed in synaptic and nowhere to be found in add/remove.
thanks again
Costas

Thank you very much, sorry for the delay to reply (one of my very busy days). I am marking this question solved.
Costas

Thanks danielgd, that solved my question.