In his case, he wanted to know how to test an application which processes audio: Reads the sound from a microphone, does something with the audio stream, plays the result on the speaker. How can you possibly test a class which reads audio from a microphone? How can you test playing sound on a speaker?
As Steve Rowe pointed out: Use a loopback cable. Play a well defined sound on the speaker and check what the microphone receives.
I suggest to move this test case into a separate test suite so you can run it individually. Print a message to plug in the loopback cable before the test and wait for the user to click “OK”. This is a unit test but not an automated one. It covers the setup steps of the hardware.
The next thing you will want to do is to check the code between the mic and speaker parts. These are now no longer dependent on the hardware and therefore simple to test.