Verify that the "Safe mode (waveOut only)" option is enabled in the QuickTime Player 7 Preferences (QuickTime 7 is installed by iTunes 6.)
 
The procedure below was tested successfully with iTunes 6.0.1.3, QuickTime 7.0.3 and QuickTime Player 7.0.3:
 
1) Quit iTunes (this is really important - the changes will not have any effect until iTunes is re-launched)
2) Open QuickTime Player 7 software (there is usually a shortcut to it in the system tray)
3) in the Edit menu, choose Preferences -> QuickTime Preferences
4) Click on the "Audio" Tab
5) In the "Devices" section at the top, verify that "Safe mode (waveOut only)" is enabled.
6) click "Apply"
7) click "OK"
8) quit QuickTime Player software
9) Launch iTunes