That's a different situation. You can try VOX to see if that works but the SCU-17 has two com ports and a din connector to the transceiver if I remember correctly. I don't think it sends audio data through the microphone like the RBA. It's a special case and has its own drivers for the operating system. Setting USB on the 1200 probably won't accomplish anything. Both Ham Radio Deluxe and FLDigi have special built-in options for the SCU-17, you might check their docs to see what they recommend.

My best and most reliable results came from FLRIG. VARA and Omnirig were inconsistent. I also ran into audio issues with the West Mountain Radio Rigblaster Advantage. DO NOT use USB-DATA as this will turn off audio input through the microphone jack. Use USB instead. This is documented in the WMR RBA manual. Also, if you are using a Rigblaster Advantage, set it to use VOX.

Are you set to USB or USB-DATA in VARA and whatever CAT program you're using? If it's set to USB-DATA, your rig may turn off mic input and expect input to come from the rtty/data line. I had this problem with my Yaesu FTDX1200 and Rigblaster Advantage (RBA). It is documented in the West Mountain Radio RBA manual, but is not easy to find (a one line statement in the middle of a paragraph). Also, set the RBA to use VOX. I stripped out all the audio components from my PC and took the RBA apart to check the jumper block before I found the statement. I hope this saves you from doing the same. Subtle, very subtle problem.