Wmmixer displays the mixer status of your computer in a small icon. Most common channels are identified with an appropriate icon. Control include a stereo (mono where appropriate) volume control and a recording source toggle button.
It is possible to change between the different channels using two small buttons (previous channel, next channel) and to change the volume of the channel.
There's nothing in the program that makes it require WindowMaker, except maybe the NeXTStep look and the fact that it properly docks. It can be used with other window managers without problems.