Mbox 2 and Snow Leopard

I have a Digidesign Mbox 2 audio interface attached to my iMac (2007 Intel Core 2 Duo + 4GB memory) which is required to run ProTools 8.0.3 LE. Since upgrading to Snow Leopard then quite often having played some audio in say Quicktime or iTunes, when I press stop the last sound made just keeps repeating over and over - like the tail of a reverb sound that never goes quiet. The only way to stop it is to go to Audio Midi setup and change clock source from Internal to s/PDIF and then back to Internal. In the Audio Midi setup window (I have an M-Audio Oxygen 8 V2 USB attached midi keyboard to the iMac as well) I've never really understood if I'm supposed to join up the In/Out little triangle connections with onscreen cables (something I used to do but only with midi connected keyboard) as both the Mbox 2 and Oxyen 8 keyboard attach via USB.