Mob: b/c you do call queue_free() in the same frame as starting to play the sound. The mob object is freed before the start of the next frame, and with it goes the Deathsound stream player, so no sound. As FreakyGoose said - create an autoload that spawns AudioStreamPlayers as needed or a separate scene hosting players for specific sounds like Fencer shows in his video (Godot 4: Audio system tutorial). Up to you.
Player:
You need to set the DeathSound stream player that’s under Player node to have Process Mode of Always (in Inspector, on the right, at the bottom under Node)