Laura Branigan: ''Branigan 2.'' (Atlantic 80052-1) After last year's exuberant hit song ''Gloria,'' Ms. Branigan has returned with an LP that proves she is not a one-hit phenomenon. Though ''Gloria'' has a sparkle that ''Branigan 2'' hasn't quite reproduced, the album has nonetheless sent ''Solitaire'' to the charts. Yet on ''Solitaire'' and on most of the other cuts is evidence of some artistic weaknesses. Her often thin-sounding voice can belt out the songs, but there's a certain lack of depth in this album. Her delivery of most songs is as predictable as the song arrangements. ''Solitaire,'' an acceptable love song, is joined by a grab bag of other tunes written by a variety of people. There's ''Deep in the Dark'' - a mildly slow-motion disco. Her version of The Who's ''Squeeze Box,'' however, is diverting, charming. Ms. Branigan deserves an ''A'' for effort so far, but we look forward to her more seasoned work.