Could you make an audio recording of these?:

- race car (hold button)
- default synth
- F0A F0B F7C F0D F0E F0F
- F0A F0B F15C F0D F0E F0F
- F0A F0B F255C F0D F0E F0F

In MAME, the default synth sound doesn't oscillate pin 13. It just goes high and stays high, that's why volume doesn't decay there.
Does it oscillate on the real device? If so, it's an emulation bug with the non-analog part.

Command C with custom synth sounds does oscillate pin 13 on MAME.