Video editing with python : adding a background music to a video with sound

You can do this with moviepy's CompositeAudioClip:

import moviepy.editor as mpe
my_clip = mpe.VideoFileClip('some_clip.mp4')
audio_background = mpe.AudioFileClip('some_background.mp3')
final_audio = mpe.CompositeAudioClip([my_clip.audio, audio_background])
final_clip = my_clip.set_audio(final_audio)