cant join 2 xvid files

[satch_1]

Hi
I have 2 problems which i think theres 1 solution for them.
first Im trying to join 2 xvid with AC3 using virtual dub but there`s a difference in the audio stream second i can join 2 xvid files with AC3 but i`ve noticed that after playing the 1 large xvid the sound suddenly got very low in the middle of the movie ( happened with 2 different movies ) , so i installed ac3 decompressor to save seperate new avi`s with the compress option then join them again but every time i hit save as an error msg appears ( error initializing audio stream compression: the requested compression not possible )
can anyone help or there`s a better way to solve this problem.

[satch_1]

Come on guys i need your help