Loudness vary greatly depending on the tone

I run the ``simple_test.py`` on a Hallowing with an Adafruit 8Ω speaker connected, and I noticed that the loudness of the notes varies greatly depending on the tone — some high-pitched tones sound several times louder than the rest of the melody.

Start by reproducing the loudness variation described when running simple_test.py on a Hallowing with an Adafruit 8Ω speaker. Inspect how the tones are generated and compare the reported volume across pitches; done means the melody plays with consistent perceived loudness across its notes.

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