I have the same issue from time to time. This may sound weird and I read it online and it actually works. Seems like small particles get into the earpiece and cause this issue, I basically sucked on the earpiece and it worked great again. I later tried a suction method with a hand held blow bulb I use the clean off my PC keyboard. This worked too. Honestly I don't think this issue should happen with this phone.
HAHA! I don't suck my phones! I blow them. But really that doesn't even help. I've had this happen over time with my Motorola Bionic but it's usually a while after owning it. Not with days-weeks."I basically sucked on the earpiece..." This brings a whole new level to a man's/woman's love for a phone. Technological erotica at its finest.
Thank God I'm not the ONLY one with this issue. I've gone though 4 Razr Maxx HD units. All have had this issue. Might work fine for a few days then suddenly if I turn the volume much past half way it will distort. Sounding like the speaker is broken. I've tried compressed air to no avail. VZW might end up having to get me into another phone if this keeps up.
Update, I have my second replacement Maxx HD coming Monday. Same problem with the crackling earpiece. I use this phone in the same manner as my Droid X for 2yrs and it never happened to it.