DIED. Mary Martin, 76, indefatigably cheerful leading lady of musical comedy during Broadway's heyday and mother of actor Larry Hagman, Dallas' J.R.; in Rancho Mirage, Calif. In the 1940s, '50s and '60s, Martin captivated audiences in a trio of theatrical triumphs, first as Ensign Nellie Forbush in South Pacific, then as the young hero of Peter Pan--her favorite role, preserved for posterity in the famous 1960 television re-creation--and as Maria von Trapp in The Sound of Music.