RIVERS PAYS TRIBUTE TO MEMPHIS  The Memphis Sun Recordings, and Last Train to Memphis  Johnny Rivers (Soul City)  3.5 Stars       Johnny Rivers, who scored numerous hits in the '60s and '70s with classic tracks such as Secret Agent Man, Poor Side of Town and Summer Rain is still in the music business via his own California-based record label, Soul City. His two most recent releases pay tribute to Memphis, the city most responsible for his primary influences in both blues and old-time rock.      The Memphis Sun Recordings takes listeners back to a magical era when rock's fledgling talents converged at an unassuming brick building at 706 Union Ave. to create the genre's most influential music. Rivers tackles essential material made famous by icons such as Elvis Presley (That's Alright Mama, Mystery Train), Johnny Cash (Big Rivers), prime influence Chuck Berry (Memphis, Little Queenie) and Carl Perkins,whose contributions to this album's Honey Don't and Matchbox were among his final recordings.      Last Train To Memphis allows Rivers to explore his roots in blues and R&B via classics like Steve Barri's Treat A Dog,Muddy Waters, Rollin Stone and John Loudermilks's Then You Can Tell Me Goodbye. Rivers also waxes several of his traditional-flavored originals like Down At The House of Blues and Joker in the Wind. Rivers, now 56, sings as smoothly as ever on both discs, sounding like he hasn't aged a day since his biggest hits. For more information on these recordings, call Soul City at (818) 980-1441.