Thread is pretty old but nevertheless...
Just in case someone comes across this in the future looking for answers.
What the issue is, is that the external speaker's magnetism picks up the metal shavings through the holes to the speaker and distorts the sound.
Best solution once this occurs is to just use a magnet to pull the shavings out and it'll bring it right back. Just be careful not to touch the speaker with it.
To prevent it: my best attempt has just been to put some masking tape over the speaker. It muffles the sound a bit but stops the shavings from getting through.
Hope this helps!