Replacing audio track in MP4 video?

[jojo8]

Have a short MP4 video clip (for iPod) with mono audio track which I want to replace with stereo audio track.

In QuickTime Player (Pro) 7.1.3, the Properties of the video file shows that the video component is MPEG-4 and the audio component is AAC.

If I select the audio track in this window, the Delete button highlights, so it seems straightforward how one deletes an audio track. But how do you add another one? What format must it be (bit rate? sample rate? etc.) to be compatible? (The stereo audio track I want to add to the video is currently mp3 format but, of course, I can convert this to AAC using iTunes or other application.)

Would like to know if I can do this edit within QT or if I need another app. Prefer 1) freeware, 2) shareware, 3) inexpensive commercial apps. Beyond that, probably it will become quickly not financially viable.

Any suggestions, step-by-step instructions, or encouragement welcome.

Thanks,
JJ

OS X 10.3.9; QT Pro 7.1.3

[jpschuck]

Would like to know if I can do this edit within QT or if I need another app. Prefer 1) freeware, 2) shareware, 3) inexpensive commercial apps. Beyond that, probably it will become quickly not financially viable.

If you alreday have Quicktime Pro, this is pretty straightforward.
1/ Open your mp3 file in QT.
2/ Choose "Edit" > "Select all"
3/ Choose "Edit" > "Copy"
4/ Open your mp4 video clip and be sure the cursor is at the beginning of the movie
5/ Delete the mono audio track
6/ Choose "Edit" > "Add"
7/ Go in "File" > "Export..."
8/ Choose MPEG-4 and click on "options"
9/ Choose "Passthrough" for the video codec
10/ Choose AAC-LC, 96 kbps, Stereo, 48 KHz, Best
11/ Save the new file at the desired location

The stereo track has to be the same length as the original mono track, else you will experience synchronization problems.

[jojo8]

Replacement audio track is shorter, so I added silence at the end to make it same length as the video.

Result is that sync problem occured -- audio arrives early before lips move.

What do you suggest I do to make audio sync with video?

In QT, there are 2 Add options: Add To Movie, and Add To Selection & Scale. I have used only Add To Movie. Should I use Add To Selection & Scale?

Thanks,
Dave

[AntnyMD]

Have a short MP4 video clip (for iPod) with mono audio track which I want to replace with stereo audio track.

Just a note that if your goal is to play the end product (after all the issues are resolved) on an iPod, the audio is going to be in the wrong format if you convert to mp3. iTunes won't sync it.

[jpschuck]

Replacement audio track is shorter, so I added silence at the end to make it same length as the video.

Result is that sync problem occured -- audio arrives early before lips move.

What do you suggest I do to make audio sync with video?
You'll have to add silence at the beginning of the audio track in order to keep it in sync with the video. Unfortunately there is no other method to guess the exact amount of silence than doing a manual fine tuning. Try something like 100ms increments.
Open the audio and the video track with Quicktime and use the menu command "Play All Sequences" (command-return) to check the synchronization.

In QT, there are 2 Add options: Add To Movie, and Add To Selection & Scale. I have used only Add To Movie. Should I use Add To Selection & Scale?

This probably won't do better : Instead of having, say, 0.1 s of constant desynchronization, you'll get variabe desynchronization throughout your movie.