Is that on Windows? By default Qt uses Windows Media Foundation backend, which does support PCM audio in WAV containers, but for some reason this format doesn't seem to be mentioned in the windows platform plugin. I don't know if it's a omission by mistake or intentional. You might want to ask the developers on the mailing list.
Qt 6.4 introduced experimental ffmpeg backend that seems to at least mention this format in the code. It's a technology preview, so you might need to compile it yourself though. Target platform and backend notes.
I'm as surprised as you that I don't have at least one codec... I'll try installing other codecs packages and have a look at VLC's Qt Wrapper but I mainly choose to use QMediaRecorder to reduce any other inclusion of librairies.
You have an example on how to use QAudioProbe in the detailed documentation of the class. As for measuring the level of a silent user, it's a completely other topic, you'll have to take into account the noise of the environment of the user.