Subtitling for music

Over a year ago I mentioned what a discovery it was for me to enjoy Mozart's Requiem Mass in D minor with English subtitles. More recently I was thinking about how the BBC make machine-readable subtitling available for their television shows but not for radio. I would like to download an episode of, say, The Curious Cases of Rutherford & Fry and be able to see subtitles as I listen, or to obtain a music CD (or download an MP3) that tells me its lyrics too. Song lyrics are already well-known; I suspect the problem is simply a lack of demand. Or, it is a difficulty of encoding subtitles within the current standards, though I suppose at least DVD tracks must make it possible to have audio and subtitles accompanied by some low-bandwidth incidental video data. I do not know if one must encode a font too or, as with PostScript^®, some may be assumed. I guess I should be grateful that British digital radio often tells me the composer and performer which is already a step up from trying to listen afterward to what the presenter says the piece was.