Android : Android: How to overlay video and audio by using ffmpeg?

I wanna make a android app which can overlay a audio to a video by clicking a button and I am using ffmpeg. The format of my video and audio are mp4 and mp3. I want the output video format is mp4. I have searched for many solutions but none of them give me a satisfied result. All of them are just replaced the original audio of my video to the audio of the audio file. What I want is to overlay my aduio file to the video file and keep the sound of the video at the same time. Does anyone have any idea to do this? Thank you!