Most have click+drag and a shift modifier for fine adjustments.

> Is it fair to assume most mouses have a scroll wheel?

Probably not, a lot of musicians develop on the go (planes etc) so they're dealing with built-in trackpads pretty often. You can still scroll but it's not as ergonomic.

This is one of the things which helped sell me on Thinkpads with their three physical trackpad buttons and trackpoint, middle click+trackpoint gets you your scroll wheel and it is quite ergonomic.

Huh, I had one of those Thinkpads and I had no idea that this was a thing!

Some audio software lets you do this but mouse wheels are incredibly imprecise compared to the mouse sensor itself so this isn't really useful for many types of control which require precise adjustment.

I think it's even more fair to assume the user has a MIDI device with a bunch of knobs on it?

If that's what they're using, why would there need to be a way to move it via the mouse at all then?