I am not sure about the music at the beginning of the promo. However, the music at the end of the promo is "O Fortuna" which is the opening movement to Carl Orff’s Carmina Burana. It is available on numerous classical music albums and has been used in dozens (hundreds?) of movies and commercials. I do not know who the performing artist is for the LOST promo. The attached link contains a longer version which also has the lyrics.