_**River Phoenix shines before passing on, plus the lovely Samantha Mathis**_ Several 20-somethings try to make it in the country music business in Nashville: Miranda Presley (Samantha Mathis), fresh from New York City; Connecticut cowboy Kyle (Dermot Mulroney); ditzy but genuine Southern belle Linda Lue (Sandra Bullock); and the bursting-with-creative-energy-and-confidence James Wright (River Phoenix). "The Thing Called Love" (1993) failed miserably at the box office but it's a pretty solid drama revolving around the music biz in Nashville. The film isn't redneck-y at all (not that there's…