find / -iname '*.mp3' -exec /home/peter/Downloads/mp3gain/mp3gain -r -c -m 4 {} \;

Control the volume using the alsa utils.

The volume of sound cards can be controlled with amixer.

amixer set -c 0 Master 25 unmute
amixer set -c 0 PCM 25 unmute

alsamixer

Save the settings.

sudo alsactl store

amixer set Master playback 5dB+
pactl set-sink-volume @DEFAULT_SINK@ +5%

amixer set Master playback 5dB-
pactl set-sink-volume @DEFAULT_SINK@ -5%

amixer -D pulse set Master toggle