Audio is not my expertise, i try my best.
The problems of your source are
volume is low
speech is fast
sound is dull
bass is insufficient
treble is high
I use the command to find the max volume
ffmpeg -i source.wav -af "volumedetect" -vn -sn -dn -f null /dev/null
, which is -6.4dB, then i transform the max volume to 0dB by using audio ...