It's not that Macs can't get a virus, it's that there aren't any (at least not very many) active viruses in circulation. And even if there were, to infect your computer, you have to grant it permission to do so.
MP3s, videos, and pictures, if they're legit, cannot carry viruses, because they do not actually have executable code. Viruses have been known to masquerade as innocuous files however, basically as simple as changing the extension. Also, there are a couple viruses that exist, that can spread via corrupted pictures (viral code embedded in the image, not executable by itself however.) That form of infection relies in old vulnerabilities in Windows Media Player, and MSN Messenger, an executable residing on your computer that is called whenever a picture is accessed, and finally... an actual corrupted picture that the executable can use to activate the viral code. I don't think any computer, outside of a lab/proof of concept, has actually been infected in this manner. There's too many requirements that need to be met, where most viruses are targeted at a single point of entry/a single vulnerability.
ClamXav is probably your best bet to get viruses that might infect any windows machines, though I'd suggest getting AVG on your windows machines/VMs.
Graphic Design / Photography / Web Design