Audio API documentation is incomplete

I've been trying to work out the kinks in a simple sequencer and after reading the Audio docs have realised that it mentions a module that is never spoken of again.

https://microbit-micropython.readthedocs.io/en/latest/audio.html

> ## Using audio

> You will need a sound source, as input to the play function. You can generate your own, like in `examples/waveforms.py` or you can use the sound sources provided by modules like `synth`.

Synth is nowhere to be found.

Also please add an example of AudioFrames from a file and steps to produce them if possible. I think PyDub might be capable of converting 8 bit mono wavefiles as AudioFrames.

Start with the Audio documentation at microbit-micropython.readthedocs.io/en/latest/audio.html and inspect examples/waveforms.py. Verify whether the referenced synth module exists, then determine how AudioFrames can be created from a file, including the proposed PyDub workflow. Done means the documentation no longer references unavailable material and includes a working AudioFrames example or clearly documents the limitation.

Written by the indexing model from the issue text.