Add multiple mp3 files to an mp4 file with ffmpeg
Use

-i /path/to/mp4File
-i /path/to/soundmp3File1
-i path/to/soundmp3File2
-map 0 -map 1:a -map 2:a
-codec copy -shortest path/to/output/fileWithSounds


FFmpeg will automatically select one video and one audio stream from among all the inputs. -map options are required to include all streams.