Libbet: Wrong note when falling in hole

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Something is distorting pulse periods on the way to the APU registers.

• platform: Firefox 63.0.3 on Xubuntu 18.04 (amd64)
• version: 0.3.2
• ROM used: Libbet and the Magic Floor v0.02

Steps to reproduce

1. Open libbet.gb

2. Start the demo by pressing the key bound to Select

3. Wait for the fourth instruction screen:

    Tiles with no exits
    contain a trap door that
    leads to the entrance.
   

4. Wait for Libbet to fall in the hole at the northeast corner.
   The game plays a descending slide whistle sound effect as she
   falls in followed by a tone as she is launched back onto the south
   side of the floor.

• Expect: The tone rings out as a D above middle C (close to 294 Hz)
• Actual: The tone rings out over an octave higher, as F (close to 698 Hz)