How would you add annotations/metadata to an mp3 file at certain points of the file, much like how SoundCloud does it? For example, if I had a 3 minute long mp3, and I wanted to add an annotation at the 1:10 mark of the file with a note of "Here's the main idea". Then mp3 players that know these annotations exist could look for them while reading the mp3 file and playing the sound.
I think you want chapter Markers?
as on http://id3v2-chap-tool.sourceforge.net/
Chapters in this context could be any of the following:
chapters within an audiobook
articles within a podcast
individual tracks within a multi-track audio file
you will need VLC Media Player, cause most Players dont show Chapters.