iTunes cannot play mp3Pro (not as common), wma, real files, ogg vorbis.

My guess is you have accidentally downloaded some files pretending to be mp3's. Virus thread: Typical virus spam file sizes - Make a note of them (click on blue link to see). (np, viruses will not affect MacOSX.)

Some audio players you can use as back up on the Mac, see Alternate Audio Players & support plug-ins. Personally I have ToolPlayer, Switch & Audion as the main ones. The first two are the best. I also have Starplayr & TwistedFlac. ToolPlayer can play a very large number of audio formats. It is only a very simple player, but it works & is good for testing audio files.