This connects to audio+Video out of sync guide
Apps to use: VirtualDub, link here
and if it's not installed: Lame Mp3, link here
Here's what to do:
If your avi has VBR you'll see this warning:

Then:
Video-->Direct Stream Copy
Audio-->Full Processing mode-->Compression

and from the list select Lame MP3, 128CBR and 48000 sample rate
(if you get an error use 44100) and OK

Now, file-->save as avi

select a save place and a save name and you'll get an avi with CBR (Constant bit rate)
Wait until the avi is saved
Done