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.

If you get the error “Not available in the current data” in the Ubuntu Software Centre, try opening a Terminal and running this:

sudo apt-get update

Update: This should also fix the error “Not available for your hardware architecture” as well