Force Windows XP to output only mono sound

Did you check Control Panel > Sounds and Audio Devices > Audio > Sound Playback > Advanced ? You can set the speakers to all possible "Mono" choices you might have. Then set the balance to 100% left. That should work.

I tried to mix the two channels and I can't in Windows Media Player or the Control Panel.

But I opened the file with VLC Player and the audio can be changed from the "Audio" menu under "Audio Device"

Try "Virtual Audio Cable":

1. Create virtual line(One will be created after installation)
2. Set it as default sound device
3. Run "Audio Repeater" from VAC folder in start menu.
4. Set your virtual line as "Wave in" and your real audio device as "Wave out".
5. Select Mono for "Chanel config" and click start, it's done!

Note: the mixed mono sound may be directed to LEFT channel only, I think it depends on device. Enjoy.