I have a java application that is taking in sound from multiple sources, and one of the abilities of the user is to record what is happening in the application to an AVI file, and I would like to include the sound in that video capture. How do I record the sound that the user would hear (ex. a result of all of the sound inputs mixed together)? I can figure out how to get the actual sound stream in to the video, but I am not sure where to get that sound source from.
Have you read the java tutorial on this? There's a lot of information that pertains to your question in Accessing Audio System Resources.
It details some recommended classes to use and provides examples of their implementation.