POP/ROCK Carly Simon: ''Hello Big Man.'' Warner Bros. 23886-1) - Carly Simon's amazing voice can hit soft, sweet highs and somber, reflective, sometimes-amused lows. It's not a voice of stunning vocal quality. Instead, it's one that's matched to the communicative demands of her songs. At times, the voice can have the hard edge of experience to it that testifies to the challenges and recollections which she autobiographically considers on her latest LP. ''Hello Big Man'' has no songs bursting with chart-scaling vigor. Instead, it's an interesting gallery of moods and reflections. The first cut, ''You Know What to Do'' has had the most airplay so far but is perhaps of the least interesting musically, and lyrics have a run-of-the-mill steaminess to them. But from there, the interest builds, with mellow ballads to upbeat reggae songs, including Bob Marley's ''Is This Love,'' the only music not written or cowritten by Simon. Her ''Menemsha'' summer romance song echoes with memories made golden by recollection. ''It Happens Everyday,'' a probable comment on the breakup of her marriage to James Taylor, has striking tenderness and generosity. ''Hello Big Man'' tells an idealized version of the romance of her parents. This LP grows warmer and friendlier with each visit.