If you want to be very precise about the whole situation, then technically, James Mercer is back, with a new bunch of friends he has hired to perform alongside him as the Shins. But such attention to detail obscures the larger fact that, hey, the Shins made some new music! “Simple Song,” their first effort since 2007, popped up on the band’s site yesterday afternoon, and though it’s not some monster, ten-minute, mini-rock-opera, it is more ambitious than the title might suggest. The wiggly guitar, some insistent piano, a major “ooo-ahh” chorus in back — the Shins (such as they are) sound bigger, almost like James Mercer spent four years not touching any instruments and then couldn’t resist playing everything, all at once, really happily. Luckily, they also still sound like the Shins, meaning any concerns you had about the new lineup’s album (Port of Morrow, due March 20) were hopefully unfounded. Good news! Okay, everyone jam quietly to the new song now.