Try setting one of the outputs high, then low then high again in a cycle with a wait of msec in between. Feed the output into an audio amp, eg one of those cheap powered speakers. That will give you a tone, to change the frequency, vary the wait time. The wait time can be a variable, so you could set it to change depending on the input conditions. You could even set the wait time as dependent on an analogue input.
However, the minimum wait time is a little bit less than 1 msec, which can be achieved using some maths and a variable, as the shortest wait block is only 1 msec. This limits your max frequency to just over 1kHz.
Another option would be to use the Crumble to trigger an mp3 player or something…
Hope that helps.