FFmpeg - Synchronize external audio file I'm creating some encoding templates for Multi-Bitrate streaming and I want to create a fallback that is audio-only. So when the bandwith of a users connection falls below x, it drops video playback and switches to audio-only.

The problem is alignment of said audio stream. I've aligned the videostream with 

-x264opts keyint=48:min-keyint=48:no-scenecut 


This creates equal intervals in keyframes, allowing consistent GOP's for streaming. But such an option is not available for audio only.

I extracted the audio from the original file, like this:

ffmpeg -i Sample.avi -vn -ar 44100 -ac 2 -ab 128k -f mp3 audio_only.mp3


That creates a seperate .MP3 file, equal to the audio-streams of the original file(.MP4). However, I have no way to ensure this is inline with the audio streams of the encoded .MP4's (the multi-bitrate set).

So, is there a way to make sure these files are aligned? Or should I extract the audio from one of the encoded files, instead of the mezzanine file?
One way to achieve this might be to create an .MP4 with your audio and an all-black image, which should encode to something not much larger than the audio file but still have the same internal structure as the full video files.