I didn't see any Audio Effects (using PE4) for mono, so I'm assuming it can't be done at the TimeLine. Probably with a plug-in it's possible.
Mono can't be set at Burn DVD.
VirtualDub allows for mono with Full Audio Processing (and can be saved as a DV-AVI, perfect for PE).
VDub can be found here:
If you try Vdub, load the file, set FileMenu/audio/full processing.
Then FileMenu/audio/conversion to mono.
Save as a DV-AVI.
This should get you there...
In PrPro, you can add a mono Mixdown Track, and via the Audio Mixer, send a stereo Audio Track to that. Similar in Adobe Audition. However, I do not know of a way to do it in PE, as I do not believe that it has full control of a Mixdown, or the ability to add a mono Mixdown Track. Unless someone has a neat little trick, I'd go with an audio editing program, like Audacity, as it should be able to handle this.
Good luck, and I'll be watching to see if there is something in PE that I have missed.
Thank you very much, I figured it needed to be done externally.