Radd, you are likely having word clock issues.. you'll need to set the external device you are connecting to your Mk3's s/pdiff connections to clock (sync) via external clock coming in from it's s/pdiff connection, if this is in fact your problem. you may also have not set up the mk3's s/pdiff connectors as one of the 3 possible active banks in the Motu PCI Audio Console, which you will find in your applications folder. that could also potentially be your problem. here's the quick fix, regardless of what your problem is: read the manual.