How to add background audio to ffmpeg video?

Question

Video

Hello. Making videos with ffmpeg. In one line I set the arguments for creating a video. The line indicates both the video and the sound that you want to add to the video. Tell me what you need to add in the line so that another background track is added, and its volume should be set to 0.5 of the full volume and its length should be equal to the length of the video

ffmpeg -i son.wav -i video_origine.avi video_finale.mpg