I suffered this infamous problem with my MH1 for almost 2 months! Sound was perfect at first, then cold weather came. However, I'm not entirely convinced moisture was the cause because I left it dry for weeks, unused even. Then one day I was brave enough to pry out the left filter + grey foam with a pin. It took a bit of time, but the only thing that remotely felt odd was the foam wasn't straight - it was angled diagonally. So I soaked it with water, stretched it out a few times, bent the opposite direction, then let it dry. Finally, I slowly re-inserted the foam and filter. Lo and behold the volume balance restored!
What made me suspect it was the foam during my trials was the left channel was brighter and louder than the right after I removed the foam & filter. I imagine because it was diagonal, it blocked both end of the left housing. When straightened out, sound must be able to freely escape at the outer ring of the housing.
