When Sound Recorder is running on a Windows Vista-based computer, the computer does not go to sleep regardless of how long the system is otherwise idle.
This behavior is intended to prevent a recording failure that would occur if the computer went to sleep during a Sound Recorder session. Therefore, Windows Vista does not let the computer go to sleep when the Sound Recorder program is running.
To work around this behavior, close Sound Recorder. Then, the computer will go to sleep as expected.