Tone examples for CLUE board need to be higher frequency so they're loud enough to hear well

The example code for the start_tone and stop_tone functions should use tone frequencies above 2000Hz.  The examples use frequences in the range of 500 - 880 Hz and those tones, when played on the CLUE board, are so low in volume that they're barely audible.  An 880Hz tone is much softer in volume than a 2000 Hz tone, and a 500-600 Hz tone is not audible unless you hold it very close to your ear.