Geoff Pullum has an odd post at the Log in which he claims that “a completely uneducated monolingual Finnish speaker” knows better how to say Sibelius’s name than the composer himself. Judging by the end of his post, he’s trying to make a point about prescriptivism; at any rate, it inspired a very interesting comment by Roger:
A Finnish friend of mine said that she would pronounce it as Si.bé.li.us. in the nominative with stress on the second syllable and a /b/, but as Sí.pe.li.uk.sen with stress on the first syllable and /p/ in the genitive (and all other cases), presumably because once you use any non-nominative case the word is Finnicized to a greater extent. She also said that she only learned how to pronounce /b/ as an adult, even though she had always thought she could. Only after hearing herself on tape could she be convinced that her b was in fact identical to her p.
That difference in pronunciation between the nominative and the other cases makes sense but is not something I would ever have guessed in advance.