How to fix this screwed part of Audio
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

Therefore,

I use the command to find the max volume
ffmpeg -i source.wav -af &quot;volumedetect&quot; -vn -sn -dn -f null /dev/null
, which is -6.4dB, then i transform the max volume to 0dB by using audio filter volume=6.4dB

I slow down the speech by using audio filter atempo=0.8

I make it lively by using audio filter aecho=0.06:0.9:50:0.7

I boost the bass by using audio filter firequalizer=gain_entry='entry(0,24);entry(250,12);entry(500,4);entry(1000,2);

I decrease the treble by using audio filter firequalizer=gain_entry='entry(4000,-4);entry(16000,-20)'


In conclusion, the command is
ffmpeg -y -i source.wav -af &quot;\
volume=6.4dB,\
atempo=0.8,\
aecho=0.06:0.9:50:0.7,\
firequalizer=gain_entry='\
entry(0,24);entry(250,12);entry(500,4);entry(1000,2);\
entry(4000,-4);entry(16000,-20)'&quot; \
output.wav

Hope you like it, good luck.