FYI: Kevin's answer is just copied and pasted from the internet.
Now to the real issue...
Malware can only exist in Portable Executable (PE) files (without your system already being compromised by existing malware which would have had to have been PE files too).
PE files include those with extensions such as:
.cpl
.exe
.dll
.ocx
.sys
.scr
.drv
So therefore files excluding those just mentioned (.mp3, .jpg, etc) >>cannot<< be infected with malware. The closest thing to an infected mp3, etc. being infected with a malware would be if a method called Steganography was used..and even then, the malware would just be embedded without even being able to be executed, so the malware is useless anyway, at most it would just be scanned as malware by an Anti-Virus program. But as I said the malware would be unable to work.