A filmmaker is asking for a pretty sizeable leap of faith from his/her audience when over one hour of their 2'40" movie consists of sung performances (FULL songs, too - that would never happen today), and just about nobody in the cast (save Keith Carradine, and Ronee Blakely - and even she only about 90% of the time) can actually sing. When Lily Tomlin opened her mouth with the gospel choir at the beginning, I nearly ditched the film, thinking "had to be there" about all those praising critics...
Luckily, I stayed with it, and Nashville, as a pure movie, is a masterful work. The dense layers of dialogue, the wide-screen panoramas, the set pieces (the car pileup "happening" being especially effective and awesome), the tossed-off one-liner/payoff ironies which are more a documenting of what everyone in the audience is already thinking than a redundancy - all of it, top-shelf and brilliant.
Also found it intriguing that Ronee Blakely, when singing, is the only character in the entire film I can recall that gets a true close-up - I guess Altman was trying to make her semi-deified, cult of personality status stand out even more.
Altman hits a bull's-eye everywhere else - how could he miss with regard to the actual singing? (The songs, themselves are pretty good for what they are.) I guess if you're going to have the actors write their own lyrics (which he did) then you're kind of commited to go all the way with it. But it doesn't make most of those performances any easier to hear...