This is a simple mp3 of a sine wave which can be used for testing:

 

• Sine Wave 440 Hz (A) for 5 minutes (mp3) - Right click for "Save Link As" (Firefox), "Save Target As" (Explorer), "Download Linked File As.." (Safari) or "Save link as.." (Chrome).

A simple test tone like this can be used with an oscilloscope to troubleshoot analog hardware.

1. Load this mp3 onto a media player or just play it on your computer.
2. Plug an audio cable from your media player to the input of the audio device being tested.
3. Start playing the test tone (loop if possible)
4. Trace the sine wave through the analog circuit till it unexpectedly disappears.  This will lead you to the vicinity of failure.