Support decoding audio/video, processing that data, and encoding to a new media file

I want to be able to load a media file, select an audio track, then receive decoded QAudioBuffers and QVideoFrames (which I can then apply audio and video effects processing to) and then feed these to a media encoder configured with a QMediaFormat.

Currently I can load an audio or audio-video media file with QAudioDecoder and receive QAudioBuffers (however I don't see a way to select which audio track to decode).
I can load a media file with QMediaPlayer and set a QVideoSink on it to receive decoded QVideoFrames. But this is oriented towards playback, e.g. it supports setting and audio output device etc. I just want to decode, so I think a new QVideoDecoder similar to QAudioDecoder makes more sense.

Better yet, a QMediaDecoder that supports audio track selection and exposes both decoded QAudioBuffers and QVideoFrames.

For encoding we currently have QMediaRecorder, but it is oriented towards capture from devices and windows, I can't feed it my own frames and audio buffers. But it must have all the encoding infrastructure needed. So we need a new QMediaEncoder that accepts QVideoFrames and QAudioBuffers and encodes them to an output location.