最主要的就是使用使用amix的filter
官方文档：http://ffmpeg.org/ffmpeg-filters.html
6.8 amix
Mixes multiple audio inputs into a single output.
Note that this filter only supports float samples (the amerge and pan audio filters support many formats). If the amix input has integer samples then aresample will
be automatically inserted to perform the conversion to float samples.
For example
ffmpeg -i INPUT1 -i INPUT2 -i INPUT3 -filter_complex amix=inputs=3:duration=first:dropout_transition=3 OUTPUT
will mix 3 input audio streams to a single output with the same duration as the first input and a dropout transition time of 3 seconds.
It accepts the following parameters:
- inputs
-
The number of inputs. If unspecified, it defaults to 2. - duration
-
How to determine the end-of-stream.- longest
-
The duration of the longest input. (default) - shortest
-
The duration of the shortest input. - first
-
The duration of the first input.
- dropout_transition
-
The transition time, in seconds, for volume renormalization when an input stream ends. The default value is 2 seconds.
这个是把当前电脑播放的声音混音到前面的文件中的例子命令（"audio="后面的改成自己实际的，ffmpeg -f dshow -list_devices 1 -i dummy可以win下获取)：
ffmpeg233.exe -re -i 1234.mp4 -f dshow -i audio="立体声混音 (Realtek High Definition " -filter_complex amix=inputs=2:duration=first:dropout_transition=0 -t 10 out.mp4 -y