Why is there a pop sound when a burning splint is extinguished in a test tube filled with hydrogen gas?
why does hydrogen burn with a pop sound?

Hydrogen is actually a combustible gas. But when you try to burn it, it explodes. However when you produce hydrogen in a laboratory, which is relatively less, and put a burning splint, it also explodes, but  in a reduced form - a POP sound. The very small explosion (doesn't feel like one, does it?) extinguishes the flame.