If you want to merge new Audio with video repeat video until the audio finishes, you can try this:
ffmpeg -stream_loop -1 -i input.mp4 -i input.mp3 -shortest -map 0:v:0 -map 1:a:0 -y out.mp4
Using -stream_loop -1 means infinite loop input.mp4, -shortest means finish encoding when the shortest input stream ends. Here the shortest input stream will be the input.mp3.
And if you want to merge new Audio with video repeat audio until the video finishes, you can try this:
ffmpeg -i input.mp4 -stream_loop -1 -i input.mp3 -shortest -map 0:v:0 -map 1:a:0 -y out.mp4
use -y option will overwrite output files without asking.