The audio system on Vista keeps track of the apps that are playing sounds (it has to, to be able to display the information on what apps are playing sounds :)). It keeps this information around for a period of time after the application has made the sound to enable the scenario where your computer makes a weird sound and you want to find out which application made the noise.
The system only keeps track of the PID for each application, it's the responsibility of the volume mixer to convert the PID to a reasonable name (the audio service can't track this information because of session 0 isolation).
This works great, but there's one possible problem: If an application exits between the time when the application made a noise and the system times out the fact that it played the noise, then the volume mixer has no way of knowing what the name of the application that made the noise was. In that case, it uses the "Name Not Available" text to give the user some information.
Windows Vista, Volume, Audio, Mixer, Volume Mixer