Are modders able to use other content from video games for their mod? Say, for example, music or sounds? I'm using some Unreal Tournament 3 music for my mod, but I don't want epic games to come after me for releasing it.

I think as long as it is not for commercial gain and doesn't cause them to lose any profit it is OK... I don't know everything about copyright laws or the individual liscencing for each game. The least you should do is read the ToS and see if it is mentioned anywhere.

In general; no one's going to care as long as you claim it as your own.

In terms of Blizzard material; they don't care if you use WoW models in wc3 for example, but they are well-known for shutting down starcraft mods created in other games. They are also very strict on people calling things "expansions" (projects have been shut down for sc because they called themselves expansions).

But otherwise you should be good as gold.

Lots of maps use copyrighted material, but no ones shutting down those.

Yeah, as long as you don't claim stuff as your own or sell it, nobody really cares.

Legally, no, you can't. It's copyright infringement. Whether or not litigation will be brought against you is another matter. Chances are high you'll never register on the company's radar as long as you're low key, not making any material or monetary gains from their property, or detracting from their products (either directly or indirectly, such as Halogen, an RTS mod based on Halo, being shut down prior to the official Halo Wars announcement by Bungie).

Giving credit where it is due for everything in your mod/map is a sign of respect, but it affords you no legal protection when permission was neither sought nor granted. At worst, you'll receive a "Cease and Desist" order from their legal department, and at best the creative minds of the company may come across your work and dig it.

In essence, it is generally not frowned upon by players to use source files from commercial games, especially if its use is to pay homage to such games, but you have no legal recourse and risk having your work gagged until purged of all offending material (the Halo RTS mod I mentioned retooled all their model names, as the models themselves were custom work, to be clear of copyright infringement).