Sounds Disappear and Reappear Randomly

I know there other sound issues reported, but I haven't seen this exact one.

All my sounds seem to be normal, except that after a random amount of time (different every time I go into a game), the ongoing sounds seem to get muffled when being repeated over and over again...as in mining out a large quantity of the same type of block to empty out a space.

When the sound fades out or goes away entirely, the block being mined is suddenly mined very quickly, as well. It does drop the proper item, but the timing is way off, and it's very startling...reminiscent of what happens in Creative, except for the actual drop happening.

It seems to be that if I switch to mining a different block (i.e. going from stone to andesite) the regular sound begins again, and the problem only happens if I mine out possibly more than a dozen blocks. Again...that number seems to be random...it is NOT the same every time.