The Song Index isn't a separate feature, it is incorporated into the library catalogue. Just search for a song as if it were a book, and you should be offered a list of books the song can be found in, click on a book title and it will show you a complete list of songs in that book.
It is not a database of songs like the DT, just an index of songs held in books in Plymouth. It's uniqueness is that very few public libraries have compiled such a catalogue, and none, as far as we know, are on the net. I've included the e-mail address as it should be possible to get the staff to photocopy and mail songs, or maybe even scan and e-mail. You'd need to negotiate with them, it's not my department. People in the UK can request any song book via ILL at their local branch library.
I can't explain the white on white. It should be green text.
this might be an easier link