I have tested this on 2 EVO's with completely "blown" speakers, they both now sound absolutely perfect. From what i can tell, it's a lot of built up metal shavings, they weigh the speaker cone down and prevent it from making the true desired frequency.
You will need a earth magnet, or a really strong regular one will do i guess,(I used a earth mag tho)
First issue: you have to take the phone apart, from there, remove speaker,
(has been covered on youtube so go a lookin)
Now.. take the speaker, sweet diagram huh?(its late) and drop it, foam side down, from about 3 or 4 inches onto the magnet. Repeat. You will see the shavings coming off the speaker and sticking to the earth magnet.
Do it for a few minutes or until the metal shavings are all gone.
Re-assemble, click your favorite player and enjoy!!
I will try to get a tutorial on youtube soon..