This says to use /dev/dsp as the audio input. But I don't have a /dev/dsp I can find my webcam's mic in /dev/snd as /dev/snd/by-id/usb-Dynex_Dynex_1.3MP_Webcam-02
Instead follow the instructions in the other question, but modify the command to:
ffmpeg -f alsa -i $ALSA_DEVICE_NAME ...
$ALSA_DEVICE_NAME is the device name displayed by
arecord -L that corresponds to your webcam. It will likely list a few options for the webcam, but picking the one starting with
plughw: should be fine.