It's been a while since I've used any of my Blackmagic Design Decklink hardware ... but I seem to recall you could patch audio to internal sound on the Mac via the Blackmagic Desktop Video utility software?
i seem to recall i had patched it to internal sound, so that the captured file has video and audio data as well. the captured files are fine. only problem is that i can't listen sound at capturing period.
my work flow: Arri cam and sound to PIX 240 via SDI cable to Blackmagic REcorder 3g via SDI to Adobe premiere pro 2019, mojave
Desktop video utility has no option to choose sound. only video input