I have an AVI file whose Italian-language audio is in MP3 format. I also have a separate English-language MP3 file. How do I replace the Italian version with the English version?

I notice that such a feature is available in VirtualDub, but only if the audio file is in WAV format.

You did need to convert to WAV format that MP3 you have.

Then set VirtualDub to use that WAV as source to recompress the video.

Wouldn't that degrade the quality by a significant amount? After all, MP3 encoding is lossy

You have to convert into wav file in order to import that stream into VirtualDub.Disable the Italian audio stream & use English audio.

Degrading ?Your ear won't notice it..

On the other hand, VirtualDubMod have the capability to import mp3 stream.

Then, dub it into avi file.

I just realized that NanDub does the job fine, although it did encode the output audio in VBR. Anyways, thanks a lot for the suggestions.

By the way, what tricks does VirtualDubMod have up its sleeves that I can't do with either VirtualDub or NanDub?

Can save into *basic* OGM & MKV file ?

Yeah that is 1 feature that what makes VDM so good too
Anyway, another way is to disable the default audio and add in your english mp3, then in video, click on direct stream copy and save it as a .avi file.