Are you using the USB B connection on the back of the mixer to connect to the
computer ot just plugging into the headphone jack and connecting that
cable to the ST1 input jacks?
For you description is sounded like you were just using the headphone jack for
the connection.
There’s no special driver needed to make that connection to the QU.
If that indeed is what your using go into the the computer sound settings
and make sure the internal speakers/headphone jack is the selected output.