I upgraded to Ubuntu 9.10 in early December, but yesterday was the first time I tried to use headphones since then.

It turns out that plugging headphones into the headphones socket doesn’t automatically mute the speakers anymore.

Here’s a fix.

Open a terminal (Alt+F2 > gnome-terminal)
alsamixer (enter)
Use the arrow keys (left/right) to highlight Headphon
Press ‘m’

This should fix it. You can now hit Esc to exit and exit the terminal.