Adding a sine generated audio track to a video stream with ffmepg
sine is a source filter, so the input format needs to be set as a filter using -f lavfi.

Use

ffmpeg -re -loop 1 -i smpte-color-bars-1080p.png -re -f lavfi -i sine=frequency=1000 -r 25 -vcodec libx264 -tune zerolatency -acodec aac -f mpegts udp://0.0.0.0:1234