Note: The version I was using was not 1.4.2pre, there was just no option for the full release at the time of this post.

Me and my friend recently updated to the full release 1.4.2 downloaded from the launcher. When we created a new world (redstone ready), the sounds for placing and mining blocks were gone. We could hear buttons placing and breaking but didn't test any further. After about 5 minutes all the sounds came back and everything was normal. Don't know if this is happening to everyone but it happened to both me and my friend.