4dots Software
CODE HELP BLOG
To add background music (with user-specified volume) to a video that already has audio, use the following code.
ffmpeg.exe -i VIDEO_FILE -i BACKGROUND_AUDIO_FILE -filter_complex
"[1:0]volume=0.5[a1];[0:a][a1]amix=inputs=2:duration=first" -map 0:v:0 out.mp4
VIDEO_FILE : video file path
BACKGROUND_AUDIO_FILE : background audio file path
You can set your own volume. 100% corresponds to volume=1 so 50% volume corresponds to 50%/100%=0.5.