Adding MP3 to Video

[brakecane]

I have a Tai Chi MP4 video in the format where it will play on my Kindle Fire 7.  I have an audio  narration of the moves in MP3 format that is time synched to the movie that I play on my MP3 player/headphones as I'm doing the moves.  I'd like to also add the MP3 to the MP4, but in doing so they become out of synch, somehow the audio lags the video. 

What should my output settings be in Avidemux?  I would like to keep the kindle format and just add the MP3.

Thanks

[eumagga0x2a]

First of all, does the issue persist also with the latest nightly? Is the audio-video offset growing? Does it help to save the MP3 audio as 16 bit WAVE (.wav) using e.g. ffmpeg or Audacity and then add this .wav as an external audio track?

[Jan Gruuthuse]

check if the mp3 has leadin, what would explain the audio shift.

Drag drop the mp4 video onto audacity, you should have one audio track into audacity now.
Now drag drag drop the mp3 on to audacity, bot audio tracks should now be visible in audacity.
See audio tracks wave representation if the same duration and start
Play and listen if the mp3 track is beyond, shift the audio track to the left [<-->]
If both tracks do match, delete the audio track from the video (1st one)
Save the new mp3 audio track.

Avidemux:
-----------
- Load the mp4 video.
- Import the new mp3 audio track

Save as new video, is audio now in sync?

If not check in audacity that mp3 is 44100 Hz, if not set to 44100 Hz and re-sample the mp3.
Still an issue? In export mp3 change Format Options to:
- Preset Standard, 170-201 kbps or Constant 192 kbps

perhaps kindle needs lower quality mp3?