Use Screencapture command to record screen -- How to get the audio source id?

The -A <id> option has been replaced by the -G <id> option. Regarding the ID, it might be SoundflowerEngine:1 or something like that. You can get all audio device IDs by running the following Swift code (requires Xcode to be installed):

import Cocoa
import AVFoundation

let session = AVCaptureDevice.DiscoverySession.init(deviceTypes: [.builtInMicrophone, .externalUnknown], mediaType: nil, position: .unspecified)

for device in session.devices {
    print("device=\(device.localizedName), id=\(device.uniqueID)")
}

# Save it as ids.swift, and
$ swift ids.swift
device=MacBook Pro Microphone, id=BuiltInMicrophoneDevice
device=Soundflower (64ch), id=SoundflowerEngine:1

$ screencapture -v -V 60 -G SoundflowerEngine:1 output.mov

Alternatively, you can use the command line tool I made without installing Xcode:

$ list-av-capture-devices
[
  {
    "deviceType" : "AVCaptureDeviceTypeBuiltInMicrophone",
    "isConnected" : true,
    "localizedName" : "MacBook Pro Microphone",
    "manufacturer" : "Apple Inc.",
    "modelID" : "Digital Mic",
    "uniqueID" : "BuiltInMicrophoneDevice"
  },
  ...
]