Tips: To convert audio only to mp3

To convert from any source to mp3 audio only:

ffmpeg -i input.xxx -vn -acodec libmp3lame -af volume=1.6 -y out.mp3

This uses default -b:a 128k. For human voices, OK to use -b:a 96k or even lower bitrates, and reduce the 48000 Hz sampling frequency down to 441000, the CD quality, with -ar 44100:

ffmpeg -i input.xxx -vn -acodec libmp3lame -b:a 96k -ar 44100 -af volume=1.6 -y out.mp3

I converted a human singing audio recorded with Audio: aac (LC) using com.android.version: 9, from 2194986 down to 1364703 in size, and sounds exactly the same to me.