One edition of a BBC programme 'The Sound Barrier' (BBC Radio 4, Sunday 17th October, 1999) included a discussion concerning the value of external information in understanding or appreciating a piece of music. Examples discussed included Beethoven's Cavatina from Op 130, which Beethoven wrote that he found one of the most distressing and difficult pieces that he had written; Mussorgsky's Pictures at an Exhibition; Stravinsky's Violin Concerto, concerning the last movement of which Stravinsky had apparently said that it was the only way he could say sorry to his previous wife. This latter example contrasted well, of course, with Stravinsky's famous saying that music can express nothing other than itself.
The interesting point here is how much one should take on board the views of, in this case, musicians, especially when those views are expressed in words. Musicians' words are often quoted with as much devotion and respect as their music, and yet why should what they say, even with regard to music be anything other than interesting. Most especially, why should they be thought to have any extra sensitivity with regard to truth?
This situation differs quite strongly from the use of extra-subject material and truth in the case of scientists.
Note also the attraction of many cognitive psychologists and scientists (esp. Pinker, Penrose) to Nicholas Cooke's 'behavioural' view of musical expression.