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EBU R128 standard is recommendation for loudness normalisation and maximum level of audio signals. The following command will take audio.wav, process it once to find the ’true peak’ and process it a second time to adjust audio.wav using the volume filter.
In addition, set output.wav to 48 kHz samplerate & 24 bit depth (Raw PCM audio) by setting -ar 48000 -c:a pcm_s24le.