I am aware that the actual sound is an intended feature, and I believe you don't understand the issue. I never noticed it in any Beta updates and believe that it has a correlation with the SMP /SSP merge.
This is really annoying, I can't tell whether or not I picked up all the XP from something I killed and I have to go back.
I haven't had this problem for about 5 official versions. It does happen occasionally in 13w04a for me, but they fix themselves immediately and are not too much of a problem.
Happens very frequently in multiplayer as well. When I created a new world in 13w04a, I was jittering in a grass block in a forest biome.
This has happened I believe since the addition of villages. Confirmed on 13w04a.
This happens for me in singleplayer and multiplayer when using any tool that has an Unbreaking enchantment on it.
I know, it's very hard to explain without a video. It's just that when you go through a portal to go to the Nether, you hear the sound that sort of makes a wavy sound while you stand in it, the one that gets louder and louder until you transfer. Well, it used to be in previous versions, that if you leave the portal before you actually transfer, the sound cuts off as it should. But now, if you leave the portal before transferring the sound follows you until it finishes it's loop.