It just sounds like the knob has come loose. There's probably a grub screw, but someone will know.

Do you hear any clunks as you change speeds? It's a mechanical process that moves the idler up and down the stepped motor pulley so it makes a fair clunk when you change speeds.

You should be able to lift the platter off (it might be quite tight) and then you will see if the mechanism is moving as you operate the switch.