用到给视频添加背景音乐，并改变音量。记录一下，与碰到同样问题的朋友共享。
import subprocess
inmp4='E:/PycharmProjects/untitled2/hecheng/191030_232_xs.mp4'
inmp3='E:/PycharmProjects/untitled2/hecheng/bg.mp3'
inmp32='E:/PycharmProjects/untitled2/hecheng/bg2.mp3'
outmp3='E:/PycharmProjects/untitled2/hecheng/bg_out.mp3'
outmp4='E:/PycharmProjects/untitled2/hecheng/191030_232_xs_bg.mp4'
cmd='ffmpeg -y -i '+ inmp4 +' -i '+ inmp3 +' -filter_complex "[0:a]volume=10dB[a0]; [1:a]volume=-10dB[a1]; [a0][a1]amix=inputs=2[a]" -map 0:v -map "[a]" -c:v copy -c:a aac -shortest '+ outmp4
p = subprocess.call(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
# 查看音轨信息
print('*'*80)
cmd='ffmpeg -i '+ outmp3 +' -filter_complex volumedetect -c:v copy -f null /dev/null'
P = subprocess.call(cmd)
记录几个cmd
# 分别降低音量后叠加 cmd='ffmpeg -y -i '+ inmp4 +' -i '+ inmp3 +' -filter_complex "[0:a]volume=10dB[a0]; [1:a]volume=-30dB[a1]; [a0][a1]amix=inputs=2[a]" -map 0:v -map "[a]" -c:v copy -c:a aac -shortest '+ outmp4 # 纯音频，叠加，可控制音量，并且设置一个循环，并按照第一个截取时间 cmd='ffmpeg -y -i '+ mp4p3 +' -i '+ inmp3 +' -filter_complex "[0:a]volume=10dB[a0]; [1:a]aloop=loop=-1:size=2e+09[a1]; [a1]volume=-15dB[a12]; [a0][a12]amix=inputs=2:duration=first:dropout_transition=2" ' + outmp4mp3 # 视频添加循环音乐 cmd='ffmpeg -y -i '+ inmp4 +' -i '+ inmp3 +' -filter_complex "[0:a]volume=10dB[a0]; [1:a]aloop=loop=-1:size=2e+09[a1]; [a1]volume=-15dB[a12]; [a0][a12]amix=inputs=2:duration=first:dropout_transition=2[a]" -map 0:v -map "[a]" -c:v copy -c:a aac -shortest '+ outmp4
ffmpeg 隐藏调试信息
-loglevel quiet
资料参考：
https://stackoverflow.com/questions/13780736/ffmpeg-unable-to-find-a-suitable-output-format-for-i
https://stackoverflow.com/questions/44712868/ffmpeg-set-volume-in-amix