PNMixer is system tray sound mixer. Running with the HighContrast theme it looks like this: Running in the tray with HighContrast theme

PNMixer currently supports ALSA and Pulse audio. It is written in C, depends only on GTK+, and does not require Gnome.

PNMixer integrates nicely into desktop environments that don't have a panel and therefore can't run a mixer applet. In particular it's been used quite a lot with tint2.

PNMixer is a fork of OBMixer with a number of additions. These include:

Volume adjustment with the scroll wheel
Select which ALSA device and channel to use (added in version 0.3)
Textual display of volume level in popup window
Continuous volume adjustment when dragging the slider (not just when you let go)
Use system icon theme for icons and use mute/low/medium/high volume icons
Configurable middle click action (default is mute/unmute)
Preferences for:
volume text display
volume text position
icon theme
amount to adjust per scroll
middle click action