There were additional scenes with Fantine but (as far as I know) none were singing. It was just an extended clip in 'Lovely Ladies', Various takes of 'I Dreamed A Dream' (you'll notice the version in the trailer is different to the version in the film) and the well-documented extended death scene.
I dream a dream we done in one take (well dicumented) There are no more versions. The trailer was just a cut of the sound over other scenes
.Unlike other movie musicals — and music videos for that matter — the vocal tracks in ‘Les Miserables’ are not overdubbed studio recordings but are the live vocal performances that were captured during shooting. In fact, most songs in the film represent one take — as musical accompaniment, piano, was piped in by ear piece. That all adds up to more pressure on the performers.
Hathaway, who was also recently interviewed by the Associated Press, discussed her fear leading up to the “I Dreamed a Dream” scene and how Tom Hooper, who won an Oscar last year for directing “The King’s Speech,” talked her through it. (Incidentally, Hathaway’s own mother played the part of Fantine years earlier on Broadway.):