Merge audio and video files with FFMpeg

If the video has no audio:

ffmpeg -i video.mp4 -i audio.mp3 -c:v copy -c:a aac -strict experimental output.mp4

If the video already has audio:

ffmpeg -i video.mp4 -i audio.mp3 -c:v copy -c:a aac -strict experimental -map 0:v:0 -map 1:a:0 output.mp4