Then you either have to play your video through QtMultimedia and use a QAudioProbe or you'll have to do it with some additional hardware.

Hi and welcome to devnet,

Qt Multimedia might not be the best framework for that task. However what you can try is to retrieve audio from both the file using QAudioDecoder and the microphone using QAudioInput and then use a library like DSPFilters to the mixing part before you output the result with QAudioOutput

Hope it helps