Fox’s SoCal soap is still a mixtaper’s dream, as this inspired all-covers set, The O.C. Mix 6: Covering Our Tracks, proves. Most of the makeovers click: Brit-hopper Lady Sovereign updates the Sex Pistols’ ”Pretty Vacant” with a feline snarl and pulsing dance beats, while Mates of State rework ”California” into a soaring lounge ballad. On the minus side, John Paul White’s pared-down ”Can’t Get It Out of My Head” extracts the sap in ELO’s symphonic prog. More often, however, Mix 6 mingles grit and smarts with surface gloss.