Or use a goertzel detector for the transmitted tone(s). It is also quite simple to generate sine waves without any trig.
Just another, less advanced, idea,
Even easier would making a really slow square wave(~0.5 Hz) that follows a simple pattern, like 80, 80, 80, -80, -80, -80, 80, 80, 80...for 5 seconds. Then one would easily be able to find the signal in the data without any advanced tricks. Just (signal<0) or not for a specified period would suffice as detection. Maybe stutter and sound quality would not be tested, however it would be a simple first step.
BTW, wouldn't stutter affect a straight line as much as it would a sound wave? If you know that the wave isn't a wave but is supposed to follow a straight line, wouldn't then all deviations be stutter? The soundsystem doesn't treat a stream of similar bytes any different from a sine wave, does it?
Of course, this will obviously not work through a microphone, but I can't see how that loop would be relevant to test. Being hardware and all.
//Nicklas