How to force ALSA output as single-channel?

Question

My speakers are broken and play at different loudness from time to time. This means that the off-center parts of music sound wrong. How can I force all sound to be centered, so that the two speakers have the same output?

I'm using ALSA.

Answer 1

You can adjust the volume of the two speakers independently by adjusting the balance. An application like alsamixer or gnome-alsamixer or alsamixergui will let you do it graphically, or you can use the amixer command-line tool:

amixer sset Master 70%,30%

This would shift the balance to the left.