This is apparently a pretty common problem, most particularly for people running Ubuntu on their MacBooks (I have a first-generation 2.0 GHz MacBook). Here is a rundown of the problem: Upon initial installation of Ubuntu 7.10 (Gutsy Gibbon), changing the master volume level, either through the slider or the volume keys on the keyboard, yields no effect when headphones or speakers are connected to the sound input jack, and when no input device is plugged in, the master volume changes the treble level.The solution that worked for me is quite simple, really. I went toSystem>Preferences>Sound, and under the Devices tab, I made sure that “HDA Intel (Alsa mixer)” was selected for the device. I then selected “PCM” from the list below. After that, using the master volume slider and the volume keys on my keyboard and on my MacBook worked as intended, whether I had speakers or headphones connected to the sound input jack or not. And when there was nothing connected to the sound input jack, the master volume changed the master volume, rather than just the treble. Very nice!All kudos to nnutter for the solution. The full bug report I referenced to solve this issue is located here.