Even Aimee Mann is tired of sounding like Aimee Mann, apparently. After four albums of midtempo grousing (extremely well-written and -played grousing, but still), the weedy singer-songwriter has gone conceptual with this ''novella'' about two hard-luck lovers crisscrossing 1970s America. Stated period musical influences include The Band, early Rod Stewart, and Mott the Hoople, and on songs like ''Video,'' the pedal steel and Dobro really do convey the rootless ache of the polyester era. But Forgotten Arm still sounds like Aimee Mann. And that's not a bad thing.