USB electric drum making no sound in garageband

USB electric drums not producing any sound. the computer recognises them (I've downloaded a MIDI monitor which confirms its recieveing the signal) just no sound is coming out. in fact when I turn on the module a little light pops up momentarily in garageband (near where it tells you time/ key signature) so even garageband must recognise its there. I use the same usb cable with my keyboard and that works fine. it's been working fine since I've got it, last few days it has stopped completely. No sound whatsoever. Pls help it's driving me insane.

The drum kit is a Alesis DM Lite. I connect it to my computer with a simple USB cable. As I said it's been working fine since I've got it so I'm guessing I must have clicked on or changed something in garageband itself so it produces no sound