This is a crazy idea, but works for me when I add a new source & no audio is coming through even though OBS says it is.
Try clicking on the "Advanced" tab of the audio source in the audio dock. Switch "Monitor" to "monitor and output" and see if that helps. Or, if you already have it set to monitor & output, set it to monitor & then back to monitor and output. If the audio comes back, woo hoo! If not, try restarting OBS.
Not sure why, but that trick works for me a lot when my audio isn't coming through. Hope this helps!