1200/9600 baud selection pertains to the received audio. From what I understand your computer hears the audio from the radio so that’s should not be an issue.
Here we are dealing with audio from Digirig to the transceiver. Still not quite sure why you don’t get audio on the air with all individual components passing their tests. For good measure you can try an audio loopback test as demonstrated here:
For the loopback you’ll need to temporarily connect pins 2 & 5 as shown in my schematic (don’t forget to mirror).