Changing volume in an audio (mp3) file with ffmpeg

I have audio of a voice lecture given by a man. I tried
ffmpeg -i in.mp3 -af volume=6 output out.mp3

It seemed to process, but the output sounded the same as the input. Then I tried
ffmpeg -i in.mp3 -af volume=20dB output out.mp3

with the same result. When I tried
ffmpeg -i in.mp3 -af volume=30dB output out.mp3

it came out louder and there was even some distortion. Is this because the man’s voice is in a low register? What can I do to just boost the audio to double the loudness? (volume=2 did not seem to do anything.)

ffmpeg -i in.mp3 -af volume=6 output out.mp3 – had no apparent effect
ffmpeg -i in.mp3 -af volume=20dB output out.mp3 also had no apparent effect