In the Qt6 documentation here, it has this line:

decoder->setAudioFormat(desiredFormat);

However, no such API is available in QAudioDecoder.

According to doris:

...the decoded audio will have the same sample rate, sample format, and channel count as that of the original media.

If you have an mp3 file of format 32k, 16bit signed int, stereo, QAudioDecoder will output PCM data of 32k, 16bit signed int, stereo from it.

...in Qt6 Beta2 there's no way to set a specific format to decode in

Therefore it is currently impossible to get predictable results from the decoder, making the decoder useless, since there is also no API to conform a media buffer to the required format.