https://obsproject.com/forum/threads/audio-crackles-with-big-sur-11-3-1.144140/
https://www.reddit.com/r/mac/comments/llbbed/capture_desktop_audio_obs_m1_big_sur/
It might need some other apps to bypass the issue.
I used BlackHole app to make OBS work, but I can not change the volumn by using volumn key on the keyboard now.
@Acer
"The macOS release of OBS Studio supports macOS 10.13 and newer."