I've put in a bit more debug-output and I quote the console output here:
Frequency : 880hz
Waveform : 63,-63 (length=2)
Frequency : 1760hz
Waveform : 63,63,-63,-63 (length=4)
Frequency : 3520hz
Waveform : 63,63,63,63,-63,-63,-63,-63 (length=8)
Frequency : 7040hz
Waveform : 63,63,63,63,63,63,63,63,-63,-63,-63,-63,-63,-63,-63,-63 (length=16)
Frequency : 14080hz
Waveform : 63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63 (length=32)
Frequency : 28160hz
Waveform : 63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63,-63 (length=64)
Six tones. Each sounds like International A (440hz) but I find it strange that each is louder.