I did a beginners' series on this a couple of years ago. The Flickr album is:
https://flic.kr/s/aHsksL3RPk
The results were impressive.
For a quick listen, the "Before" audio is here:
https://flic.kr/p/Ee3Z9M
And the "After" audio is here:
https://flic.kr/p/F8K7nW
I checked the speaker, and it doesn't seem to be the problem.