Is it possible to map mouse buttons 3 and 4 to control system volume on a Mac? KeyRemap4Macbook doesn't seem to have the functionality, nor SizzlingKeys. I have them both. Is there some way to manually bind these buttons to control system volume?
What kind of mouse? Is there a proprietary driver for it? Kensington has a driver for their mice & trackballs that can assign their buttons. Before I had an effective Kensington driver, I used USB Overdrive. It works with a lot of multi-button HID devices.
If mapping the buttons to perform key press F11 (volume down) and F12 (volume up) doesn't work, then i'd suggest the following application.
ControllerMate (commercial ~$25 USD) it allows you to map almost any input device button from mice, non-standard and standard keyboards, and game controllers, to specific tasks (and they can be as complex as you want).
The image below is from their website and shows an iTunes Volume control created from their application. For me this application blows USB Overdrive out the water if you need the features it provides.
A list of known compatible devices is available from their website here if you want to see if your device is listed and supported.
A demo version is available here