Updated for VLC 2.2.1:
- Tools > Preferences > Audio > Output. Select ALSA Audio Output.
- Tools > Preferences. Select the "all" radio button at the bottom.
- Audio > Output modules > ALSA. Select the mono option from the Audio output channels dropdown menu.
- Restart VLC
Original answer:
You can use VLC to do this. But I had trouble getting the mono to actually work. Here's what ended up working for me:
- Tools > Preferences > Audio > Output. Select ALSA Audio Output.
- Tools > Preferences. Select the "all" radio button at the bottom.
- Audio > Filters. Select the mono option ("audio filter for stereo to mono")
- Restart VLC