Not to mention that the codec barely tells you anything useful on its own. There are many parameters that affect the quality and file size. And heck it's not even guaranteed that you will be able to watch a video just given its codec. Like an old computer might be able to decode H.264 but only be able to do it in real-time at 480p due to processing requirements for going higher than that.
Audio patents are expiring and there shouldn’t be any difficulty decoding real time most audio codecs. If you can decode the video real time there’s a very good chance you can decode the audio real time too.
Though things are pretty boring and stable right now, there have been many times when audio codecs were changing rapidly and many people didn't have (and often couldn't get or use) an audio codec that was being commonly used in video files distributed online.