Using this command to record a 15 second screen recodring of a webpage with and embedded video with audio:

ffmpeg -f avfoundation -i "1:3" -preset ultrafast -r 25 -t 15  out.mkv

avfoundation devices:

[AVFoundation input device @ 0x7fa2f6c0f280] AVFoundation video devices:
[AVFoundation input device @ 0x7fa2f6c0f280] [0] FaceTime HD Camera
[AVFoundation input device @ 0x7fa2f6c0f280] [1] Capture screen 0
[AVFoundation input device @ 0x7fa2f6c0f280] AVFoundation audio devices:
[AVFoundation input device @ 0x7fa2f6c0f280] [0] Soundflower (64ch)
[AVFoundation input device @ 0x7fa2f6c0f280] [1] Movavi Sound Grabber
[AVFoundation input device @ 0x7fa2f6c0f280] [2] Built-in Microphone
[AVFoundation input device @ 0x7fa2f6c0f280] [3] Soundflower (2ch)

This results in very choppy audio, but fine video. Also tested this with the internal microphone, same problem, and a local video file, which also had similarly bad results.