How to keep audio and video in sync? I have a few videos I have to convert for a website. They are PowerPoint slides with a narration track over top. The originals I have to work with are in m4v format. I have converted them to mp4 and webm, but it seems like the video in the converted files plays too quickly. The audio doesn't match up. I used ffmpeg to convert to mp4 format. Is there an option I could pass in to keep the audio and video together?
It sounds like the video frame rate probably got messed up.  You need to make sure you compress the video to the same framerate in webm as the source is.  Also, m4v and mp4 are synonyms, you don't need to alter the file, you can just change the extension.