<div dir="ltr"><p style="margin-top:0px;margin-right:0px;margin-left:0px;padding:0px;border:0px;font-variant-numeric:inherit;font-variant-east-asian:inherit;font-stretch:inherit;line-height:inherit;font-family:-apple-system,BlinkMacSystemFont,"Segoe UI Adjusted","Segoe UI","Liberation Sans",sans-serif;font-size:15px;vertical-align:baseline;box-sizing:inherit;clear:both;color:rgb(35,38,41)">After a lot of research, the best approach I could find was by splitting the problem in two steps:<br><br>- Create a method to mix all the audio files you need using filters and then, export an output file using the format WAV (<a href="https://github.com/xtingray/audio_mixer/">audio_mixer source code</a>).<br>- Create a method that transcodes the WAV file generated in the previous step, into a MP4 file (<a href="https://github.com/FFmpeg/FFmpeg/blob/master/doc/examples/transcode_aac.c">AAC transcoding example</a>).<br><br></p><p style="margin-top:0px;margin-right:0px;margin-left:0px;padding:0px;border:0px;font-variant-numeric:inherit;font-variant-east-asian:inherit;font-stretch:inherit;line-height:inherit;font-family:-apple-system,BlinkMacSystemFont,"Segoe UI Adjusted","Segoe UI","Liberation Sans",sans-serif;font-size:15px;vertical-align:baseline;box-sizing:inherit;clear:both;color:rgb(35,38,41)">PS: I am aware that including the transcoding process as a filter would be a very fancy solution, but I couldn't find any reference of how to do it.<br></p></div>