In the NUnit GUI, when you click tools then on options, can you put in two new options - play sound effect for successful build and play sound effect for failed build. These would simply be two label controls, two text box controls, and two browse... buttons. The options allow you to browse to a .wav file on your local pc that will be played by the NUnit GUI for successful builds and failed builds. For example:
Success Sound: (txtSuccessSound) (browse...)
Failed Sound: (txtFailedSound) (browse...)
When the ok button is clicked in the options dialog, the path to the .wav files are saved in the options of NUnit GUI. When a successful build happens, if the Success Sound is populated, it is then played, otherwise, it is not played. Likewise, if the failed sound effect option is populated, the failed sound effect is played for a build that fails.