In conversion I missed these arguments were being passed, but noticed
it when an implausible value for audio_channel was sent to mp3's
reset_buffer method.
It's not clear whether there was any negative impact to this, but it
should be fixed!
When nrf pwm audio is introduced, it will be called `audiopwmio`. To
enable code sharing with the existing (dac-based) `audioio`, factor
the sample and mixer types to `audiocore`.
INCOMPATIBLE CHANGE: Now, `Mixer`, `RawSample` and `WaveFile` must
be imported from `audiocore`, not `audioio`.
