The website for Sweeney Todd has just been updated, and it contains a number of audio selections. I'm hardly alone in being simultaneously excited by Tim Burton directing my favorite musical and skeptical of a cast made up largely of people who are not known for their singing.
The clips on the site, though, are heartening. Most are of Johnny Depp and Helena Bonham Carter. Neither will ever be mistaken for powerful singers, but they're not atrocious. (Alas, no Sacha Baron Cohen yet.) These versions of the songs are a bit thin on their own, and sometimes the actors get overwhelmed by the lush orchestrations, but I can imagine the songs working pretty well on film, which, thanks to the way the camera modulates the audience's proximity to the actors, can be much more effective as an intimate aural environment than live theatre (or maybe it's just me -- I hate plays where the actors are heavily miked, and I have more than once walked out of shows because of the sound design). Here's a fun YouTube comparison (audio) of four versions of Sweeney (Len Cariou, George Hearn, Michael Cerveris, and Johnny Depp).
The songs promise to be streamlined in the film, and that's not necessarily a bad thing, since most of the worst movie musicals are the ones that stick too faithfully to the stage version. I love "A Little Priest" in live performance, but if the whole thing were in the movie, the pacing would be quite a challenge, given how static and word-based the song is.
In any case, some of my fears about the movie being embarrassing or unintentionally cringe-inducing are allayed, and I'm very much looking forward to seeing Burton's visual interpretation of it all.