How to intentionally get mono sound output?

[Edgars Šmits]

My son runs Ubuntu (Gutsy) on his laptop and does DJ work on weekends.
He wondered if it was possible to get mono output from his laptop
occasionally - he uses Amarok, there seems to be no such option for
playback. I searched through the list archive, the only mention of
mono is when there is a problem, not really the issue here, stereo
works great, and in the forum one guy did ask how to get mono as he
listens with one earphone only, the only response was an Audacity
setting change...

Any ideas for a simple solution - how to optionally get mono output?

Thanks

ED