In SSP and SMP some sounds do not play.
For example you can hear cows trample and moo and also hear floating water, but
no movement sounds an block hit sounds. (Block breaking sounds are allright).
After some time the sounds return. A few minutes later they are gone again.
When you change the music and sound volume the bug reappers.
I'm seeing the same I think. I've had a wide variety of sounds sometimes failing to play, including a creeper explosion (all I heard was the 'crack' noise from taking a heart or so of damage), chickens' death squawks, the eating sound for food, the level-up sound, the sound for picking up items, sheep eating grass, shears cutting a sheep's wool, dirt not making a noise when placed, and one or two others I don't currently remember. I haven't seen any pattern, and I haven't tried changing the game volume at all, even when this issue has occurred. And I'm on Mac OS 10.5.8, not Windows.