- Download avidemux, run avidemux & open .avi file
- In avidemux select Audio | Main Track… & select External mp3 as the video source
- Select the (output) format on the main screen and save file giving it the correct extension
- I could not get the shift to work to start the mp3 from a certain spot so I downloaded audicity. Open the mp3 in audicity, set it playing and select a section with the mouse and then select Edit | Trim to silence the non selection and resave as a new nmp3.