I've been beating my head against this for a while and I've run out of time and patience, so dear Lazyweb, please lend a hand...

Something changed recently in Youtube such that many of the higher resolution video formats are no longer listed in url_encoded_fmt_stream_map but must be extracted from the dashmpd file instead. But the MP4 URLs I am finding are giving me 403 or 404 errors. So I'd like you to help me figure out where it's going wrong.

1. Get HTML from https://www.youtube.com/watch?v=RlaoFHe6JL0
2. Find the "dashmpd" URL, which will look like https://manifest.googlevideo.com/api/manifest/dash/...
3. Note that it contains /signature/ rather than /s/ which means indicates that it is the real signature, not an enciphered version of it.
4. Find the <Representation id="137"> in there.
5. Its BaseURL should be a downloadable audio-only 1080p MP4, but instead it is 403 or sometimes 404.

What am I missing?

Update: Uh maybe I fixed it maybe.