Comment 52 for bug 201957

I installed Karmic on my MacBook2,1 and was having the same problem, but I just ran alsamixer and messed around until it started working. Sorry that I don't have more of a concrete answer, but perhaps someone else can confirm what I *think* remedied the problem:

If you start alsamixer and tab until both playback and capture devices are listed, then scroll over as far right as possible, you should have Mux, Mux1, Speaker, Speaker1 on the right. I think I just increased the vol for Mux1 and Speaker1 (making sure they were both unmuted, of course). After that, transitioning from speaker to headphones worked as expected.

If you get it working this way, don't forget to 'sudo alsactl store' to keep your settings.