I had this problem when I installed the new QuickTime 3 (http://www.apple.com/quicktime/download/index.html) The error I received on my Mac was:

     "You may experience problems playing a video track in "blah.mov" 
      because the required compressor could not be found.  Would you 
      like to close the movie or continue?"  

Don't forget that if using QuickTime to view movies is something new to you, you need to set your browser (i.e. Netscape) up properly. For Netscape, select "Options/General Preferences" from the menu, then select the helpers tab. Now you can set things like "QuickTime Video" (left column) to "QuickTime plug-in" (on the right) by using the edit button.

If you would like to see QuickTime play an AVI, you need to set Netscape correctly also. Select "Options/General Preferences" from the menu, then select the helpers tab. Scroll down until you see the "AVI to QT quality" utility in the right column. Select it, then click "edit" to edit the entry, checking off "QuickTime plugin" rather than the "AVI to QT quality" application.

If you are using Netscape, first check your settings under Options/General Preferences/Helpers. If you are using QuickTime, you can either use the QuickTime plug-in option, or if that doesn't work, try using QuickTime's Movie Player - remember if you are using Movie Player for an audio file, you must set the file type. The default will probably be "MOov" - a setting you only want to use for movies.

You could also try going to http://download.com and searching for the application called SoundApp - it can play any number of different audio files.

Here is a great reference page where you can test out your browser's compatibility with different types of audio files - JerMidi's Browser Audio Test Page (http://www.our-town.com/~jermidi/otaudio.html).

If anyone has other suggestions for PC users, Internet Explorer users, or have had audio or video problems and have different solutions than those listed here, please email me and let me know what they are!