Weird sound happening often in Minecraft: "minecraft:none"

Sometimes the attached sound (minecraft:none as confirmed in the 1.7.10.json file) occurs e.g. when I spawn a Ender Dragon in the Overworld, when I try to make it enter one artificial Ender Portal (in Creative). Also when I see the "Game Output", even after exiting the game normally, the following is shown:
[Client thread/WARN] [STDOUT] Unknown sound tried to play: "minecraft:none"