Changed to using Samples\MfTranscode\MfTranscode sample 3 (same issue in sample 2). This will only affect Delphi >= 10.1 as it's in an IFDEF
In dlgAudioFormats.pas you reference sgResolutions, this actually needs to be sgAudioFormats
Anyway, having made these changes and opening a re-named m4b (m4a is OK so used that) I now need to convert it so a 16Khz WAV. Why a WAV? Well it's going to be input for whisper.cpp
The problem is that it won't then transcode to a WAV. GetWinAudioEncoderFormats calls MFTranscodeGetAudioOutputAvailableTypes which says nope...
And WAV is definitely a supported type (#80)