Problem joining 2 pieces of video with different audio codecs



Problem joining two pieces of video from different sources with Videoredo with different audio codecs:

I have a film which I captured from the digital tv and this is encoded with audio of codec AC3 @ 48000 sample rate. The picture quality is superb. Unfortunately the first 4 minutes of the film are missing because I was not aware that the film was on in time. I added this part successfully to Videoredo’s joining list with no problems.
I then took the missing 4 minutes from another source which had been captured from an old VHS source and written to DVD and tried to add that to the Videoredo’s joining list. However it refuses to accept this part because it is audio encoded as audio codec MPEG and sample rate 48000.
Both the parts were actually created using Videoredo as MPEG2 files.

Does anybody know how I can change the 4 minute clip to audio codec AC3?