I had just replaced my stock silver bezel with a black one on the Bold. After I installed it, there was an extreme amount of feedback (high-pitch noise) coming from the speaker phone. So much, that it was pretty much unusable.
I had a closer look at the inside of the stock bezel and found that there are 2 tiny mesh screens glued to it - one at the top by the speaker, and one at the bottom by the mic.
They are VERY fine and practically invisible.
I disassembled my Bold and was able to "transplant" them to my new bezel using a pair of fine tweezers and pressed them in place, as the adhesive was still on the screens.
I put it back together, and Voila! No more feedback!
I would imagine that if even one screen gets damaged or punctured, you'll have the same problem.