Synthetic original source, not speech.
ffmpeg -i source.mp4 -map 0:a:0 -af volumedetect -f null -
mean_volume: -24.1 dB
max_volume: -20.6 dB
FFmpeg volumedetect sample-based measurements for a synthetic tone after decoding; not true peak, integrated LUFS, speech intelligibility, human listening or a LinkedIn loudness target.