Regarding your notes: it may be independent of all those things but it may be dependent on the compiler toolchain used, the Qt version in use, and the Windows version for both recording and playback.
The Windows component that generated the audio stream provided something odd that Windows Media Player cannot understand. ffmpeg produces valid AAC Low Complexity the Windows Media Player does understand. It's unclear what the original stream is. If the original stream was AAC Main Profile then ffprobe would have reported "Audio: aac (Main) (mp4a / 0x6134706D)...". High Efficiency AAC would be reported as "Audio: aac (HE-AAC) (mp4a / 0x6134706D)" (and less likely to be playable).
Does the audio play correctly if extracted from the container?
ffmpeg.exe -i original_video.mp4 -c: copy -vn original_video_audio.m4a