Mouse Preferences

From Audacity Manual

When using one of the Audacity tools, a number of tasks can be conveniently performed with the mouse by simple click and drag gestures.

If your mouse has a standard scroll wheel or scroll ball, you can also use the mouse for vertical or horizontal scrolling of tracks on the Audacity screen, and for horizontal zooming in and out on the waveforms. The image above shows a complete list of the available mouse bindings, but currently these are not configurable by the user.

Mac users: CMD-Wheel-Rotate (the equivalent Audacity binding on Mac for zooming in and out) will only work if you disable "Zoom using scroll ball while holding Command" in the System "Mouse" Preferences.