This is purely anecdotal... but I find that when you smoke crystal that is 'burnt, after a while you'll develop somewhat of a 'cloudy' voice. It almost sounds like there's smoke stuck in your throat, and your vocal chords will hurt a little. If I already have a little cough and I smoke, it will slightly irritate it but not as much as smoking cigarettes or weed.
Again, I only noticed that side-effect when I burnt a couple of puddles and still smoked them. If there's a gross, burnt plastic taste when you're smoking and it's all black, you should probably just clean it off and start a new one.