This makes all the samples from Dan's collection register as 440Hz
when playing on pwmio or i2sout, using https://webaudiodemos.appspot.com/pitchdetect/index.html
to detect the frequency played (all should show as A 440Hz; an error
of up to 20 "cents" should be treated as OK)
There's an audible carrier with PWM output and the 8kHz samples. This is
probably a limitation of the peripheral which is documented as being for
input signals of 44 kHz or 48 kHz; the carrier frequency is a fixed
multiple of the sample frequency.
Closes#7800
cb5e1a1e98