Hi,
I had the same problem and figured out, that i can shortly (without restarting OBS) got to properties and switch the audio sources for a second, then go back to my needed audio source.
Totally annoying...cause my chat had to yell at me everytime that the sound is off.
And it was completely random but always in the first 1-15 minutes.
BUUUT, what really helped, i always had the twitch app opened to communicate and have a look at the moderator view while streaming (with muted live video in the app), and since i did not open the app at all i am not having a single problem with sound. Cannot explain 100% why, but the issue is gone without opening the twitch app on the same Mac where the stream is.
tested it many times since and everything is running smoothly, soundwise!
Hope this also will fix your problem
Greets
IKO