The ding for the Enter is part of Windows and not MediaLab. To control these sounds, you need to go to Control Panel > Sounds. Once there, click on the Sound tab to find the individual sounds for Windows. The "ding" in this case seems to be the one named "Default Beep."
The simplest solution here seems to be to select "No Sounds" from the Sound Scheme drop-down menu. It should still let MediaLab work but not allow for the "ding."
If you wanted to remove the "hit Enter to continue" message, you can do this by putting the attached file named substitute.txt in the same file folder as the rest of your MediaLab .que and .exp files (for more information about this feature, please refer to this section of the MediaLab manual detailing the substitute.txt feature: http://www.empirisoft.com/medialab/h...prefs_messages)