iTunes can not play ALL file types. It cannot play mp3pro files which also have the .mp3 extension. It will not play wma or realplayer files or ogg files (but can with 3rd party extension.) See How to transfer music to my iPod (& iTunes) for mac users & get Audion 3 which can play both ogg vorbis & mp3pro files. If it still can't play them then there must be something wrong with the file. In that case see T3 Spammers; mp3 Bit-Rates Versus File Sizes (click on link)