Good question, the codes are somewhat buried. You will not find out what 973 is specifically, but it falls under the 9xx branch of error codes outlined in the media.h source here.
What's really interesting is the 9xx errors are the only ones not referenced in the documentation, but they are labelled MEDIA_INFO_TIMED_TEXT_ERROR
.
The description is that the player:
Failed to handle timed text track properly.
Now what that means is also a good question to which I don't have the answer :)
But I guess either it expects or cannot parse some kind of timed text track with the music file itself. Is it an mp4 or 3gp by any chance? There is some more info in this SO question. But I believe it's related to a subtitle track in the media, which wouldn't make any sense for a purely audio track, but maybe there is an .srt file or similar in the vicinity of the track it's trying to parse.