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ROCK/POP Sondheim, Stephen: ''Sunday in the Park with George.'' (RCA Red Seal HBC1- 5042.) - When reviewers of Broadway musicals talk about ''accessibility'' in terms of a particular composer, you know you may have to work to get his musical message. So it is with Stephen Sondheim. Sondheim is no Richard Rodgers in the showy show tune department - although he has produced some very listenable music. Nonetheless, his ''Sunday in the Park with George'' - a superb musical experience - is not easily taken in at once. There's Mandy Patinkin's sensitive, intense George, Bernadette Peters's brassy Dot - along with RCA's crystalline sound. But the subtle melodies, nuances of mood and emotion, and brilliant color of this very original work aren't rewarded in a casual listen. Vivid splashes of sound, numerous mildly dissonant passages - and the sadness, longing, and humor of Sondheim's lyrics - come in ingeniously together to create a satisfying whole. The enclosed libretto - and gallery of pictures reproduced within the jacket - help make the LP a full theatrical experience.