This feels like a hypothetical problem only. In practice, the use of .mp4 for audio is rare. Users can rename the audio file’s extension to the established convention of .m4a in these cases. In rare cases where this is not possible, HTML can still be used to embed the file. But this objection was already discussed at length earlier in the topic, so I’ll stop. If we introduce new syntax for audio and video we’re putting an additional burden on users to learn the new syntax, which is an actual problem as it increases complexity; I don’t think this edge case outweighs that.
Internally displayed content (such as images) that is embedded in the document is the opposite of external context that remains outside of the document (links). The exclamation mark before the link syntax []() means “not”, so ![]() represents content that is the opposite of externally linked content. With this is mind, I think it’s in keeping with the meaning expressed by Markdown’s syntax to extend the ![]() syntax to other kinds of embedded content besides images.