The QuickTime Player included with QuickTime 4 seems to open some MP3 audio files but not others. Why is this?

QuickTime 4 supports the majority of available MP3 audio files. QuickTime 4.1 can also play MP3 files encoded with VBR (variable bit rate). QuickTime 4.0.x only supported CBR (constant bit rate) MP3 encoding. To play MP3 files with VBR encoding, you should upgrade to QuickTime 4.1.

See article 60690 " QuickTime: How to Upgrade To QuickTime 4.1 " for more information on upgrading to QuickTime 4.1.