I purchased a speaker for $7, and replaced it, but the sound quality didn't improve, so I removed the grill (a part with no electronics but some adhesive) and dipped it in rubbing alcohol for an hour. Then I scraped off excess gunk with a toothpick. The sound quality is great now.
To get to the grill, I removed the speaker, which is a bit of hassle. Maybe you can remove the grill without removing the speaker, but I wasn't able to.