As far as I know, Firefox doesn't have a built-in setting to assign Ctrl+Mouse Wheel to audio volume. But can turn off the zooming behavior so the wheel just acts like a wheel whether Ctrl is pressed or not. Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste with_c and pause while the list is filtered

(3) Double-click the mousewheel.with_control.action preference and edit the value from 3 (zoom) to 1 (scroll), then click OK

YES! Thank you once again jscher! You're a hero! :)

Just to clarify, my volume control is assigned with independent software. It works again now! :)

MartyJames said

I've just found that if I do Ctrl + mouse wheel, the page now zooms! I've used Firefox for about 15 years and never had this happen before. The only thing that's different today is that I have just uninstalled Flash? How do I disable this? I have always used Ctrl+mouse wheel to control the volume. For me, it is essential to be able to quickly adjust the volume with the mouse wheel, and very frustrating not being able to!

I guess I got a mouse problem!