Hi. I'm David, and I'm happy to help you.
Could you clarify whether you are referring to audio output (sound) or video output (display/screen)?
If it is sound, Windows often automatically routes audio through HDMI/DisplayPort when a second monitor is connected. If it is your display, it might be a projection setting issue.
Please let me know which one you are experiencing so I can give you the exact steps to fix it.
Looking forward to your reply.