A well-functioning worm bin smells like fresh forest soil, with perhaps a hint of coffee or banana, just what you fed to your worms last.
Compost worms eat your waste as soon as it starts to rot. Before the bacteria that cause the stench of rotting fruit can develop.
Only if you feed your compost worms too much waste can it start to smell. There is then so much rotting waste that the worms cannot handle it, and if there is also insufficient oxygen available, the anaerobic bacteria have free rein. The solution is: remove some excess waste, and add some extra sawdust or cardboard.