You should first check wich sources are available. Because x11grab is not available by default:
ffmpeg -sources
man ffmpeg provides more help in this case…
Additional some important informations are missing:
What are you doing and how? You try to do a screen capture? Which sound do you try to record? Where is the sound coming from? An additional player or browser or what? Are you playing a video in an unknown player and try to capture the sound?..