Simple enough, a slime killed me on hardcore mode and I can still hear them. On the title screen. After deleting my world. While the Minecraft window is minimized.
How To Recreate:
1. Create a hardcore world (mine was a superflat)
2. Get killed by a mob that makes noise. (Like a spider or a zombie.)
3. Delete the world (on the worlds screen) then go to the main menu.
4. Minimize Minecraft.
You should still hear them.