@Pjmcb123 if you search Nobsound Little Bear MC102 on Amazon you will find the switcher I use. It costs approx £25.
I plug one channel of the MC102 unit into the headphone out from the prime go and the other channel is connected to the headphone out from my sound card (I use an iRig stream or Behringer UCA202). I then connect my headphones to the MC102 unit and this way I can hear the master output in my headphones.