I had issues with this. If the sample rate (48khz vs 44.1khz etc) is not the same, the sync will just fail and you won't know why. You have to make sure the sample rates of both sound sources are the same first. I did this via Reaper, which can import all kinds of things (including video, though only the audio will be visible to you if you don't set it to show the video window) and export to basically any format (including video formats like mp4, though you can just strip out the audio as well). The typical "standard" for video is 48khz, while audio is 44.1khz. Of course in reality it depends on the particular device, manufacturer, settings, etc.--there really isn't a true standard these days.
