Question / Help mkv with 2 audio tracks to mp4 with mixed 1 audio track

[vitecp]

Looking for help with this question
I have one mkv video with 2 audio tracks in it.
How can I easily convert it into mp4 with 1 audio track (audio should be mixed together) without video reencoding?

Any thoughts?

 

[Harold]

Mix the audio tracks in an external program then use ffmpeg to copy the video and mixed audio tracks to a new file.