Does lowering the volume count as remixing?

Because it says in the creative commons thingy, that if you edit/change it you get to keep it.

Probably not. Why do you want to take credit for someone else's work?

Nah, just asking.

less hassle of making slides

...what do you mean, "you get to keep it"? If a Creative Commons license says you have to attribute the thing to its original creator, then no matter what you do you still must credit the creator. (If it says no derivative works, then you can't do anything to it.)

However, I don't think (uniformly) altering the volume of a piece of music or some other sound would count as creating a derivative work.

Altering the volume isn't considered to be altering the song, simply because you can turn up the volume and hear the original. If you change the volume and as a side effect, the song changes, then its not allowed anymore. The entire idea is about recognision. As long as you can compare the song to the original, and it seems to be the same, its considered the same. The volume doesn't change that, as people will still say: its made by that person, not by you.

Remix: to mix and re-record the elements of (a musical recording) in a different way.

By that definition, no, adjusting the volume does not count as remixing, because you're not mixing the elements in a different way. After all, like LPChip said, the song's still exactly the same.