Mouse Preferences
From Audacity Manual
The image above shows a complete list of the available mouse bindings (the actual window in the program has to be dragged down to show the entire list without scrolling). Currently, mouse bindings are not configurable.
Most of the mouse bindings are for click and drag gestures when using one of the Audacity tools.
If your mouse has a standard scroll wheel or scroll ball, you can also use the bottom three gestures listed above for vertical or horizontal scrolling of tracks on the Audacity screen, or for horizontal zooming in and out on the waveforms. Horizontal scrolling and zooming always require the SHIFT or CTRL key respectively to be held down, irrespective of the pointing device.
Mac users: CMD-Wheel-Rotate (the equivalent Audacity binding on Mac for horizontal zooming) will only work if you disable "Zoom using scroll ball while holding Command" in the System "Mouse" Preferences.