Off Broadway. A sound of musicals.

''I Hear Music...'' introduces the title of one of the most engaging attractions to arrive Off Broadway so far this season. Without stretching things too far, the title might apply to Off Broadway as a whole, where the sound of music is frequently heard. Of slightly more than 40 current Off Broadway productions, 16 are musicals. (Like political opinion polls, theatrical statistics vary from week to week.)

The complete title of the show referred to above is: ''I Hear Music ... of Frank Loesser and Friends,'' now tenanting the Ballroom on West 28th Street. In the course of two stylish acts, svelte Jo Sullivan sings music associated with her career. About half the numbers are by her late husband, Frank Loesser. The rest are by the likes of Kurt Weill, the Gershwins, Charles Strouse, Johann Strauss, and Puccini.

Miss Sullivan intersperses her songs with incidental reminiscences: being coached by Lotte Lenya for ''The Beggar's Opera,'' life at the New York City Opera Company, and the ordeals of auditioning for ''The Most Happy Fella.'' The singer created the role of Rosabella in that 1956 Loesser adaptation. In ''Standing on the Corner,'' one of the excerpts from its marvelous score, Miss Sullivan yields the stage to her five instrumental accompanists, who prove most happy fellas with the deft Loesser lyric.

Although Miss Sullivan refers more than once to the Loesser dictum that ''loud is good,'' the lady herself proves that sweet and lyrical are lovely. In the course of an eclectic program, she runs the gamut from ballads like the Kern-Hammerstein ''You Are Love,'' the Duke-Harburg ''What Is There to Say,'' and the Strouse-Lerner ''Dance a Little Closer'' to the light, lively, and even comic. Among the latter are ''When the Red Red Robin Comes Bob Bob Bobbin' Along,'' ''The 88 Rag,'' ''I Believe in You, '' and ''No Swallerin' Place,'' an unpublished Loesser song. The music is superbly played under the direction of pianist-arranger Colin Romoff in a performance adroitly staged by Donald Saddler. ''I Hear Music...'' makes delightful listening.

By far the most ambitious of recent Off Broadway musicals is the revival of the 1976 Sondheim-Weidman-Wheeler extravaganza ''Pacific Overtures,'' at the Promenade Theatre. Even with a much reduced orchestra and with the doubling and tripling of parts among the cast, what Stephen Sondheim calls a ''Kabuki vaudeville'' remains a work of large proportions. Adopting certain techniques of the Japanese theater, ''Pacific Overtures'' fictionalizes the consequences of East meeting West when Commodore Perry and his four black ships visited the island empire in 1853.

In his book ''The World of Musical Comedy'' (Barnes), Stanley Green notes how Mr. Sondheim showed his previously demonstrated ability ''to write in an adopted style while still retaining his own individuality of expression.'' Sondheim found that he was able to relate the Japanese scale to his own style ''because suddenly it had a Western feeling and at the same time an Eastern feeling.''

''Pacific Overtures'' contains some of Mr. Sondheim's richest and most inventive composing. Its songs can be evocative, as in the opening paean to tradition, or menacing, as in ''Four Black Dragons.'' ''Chrysanthemum Tea'' literally dramatizes a murder in the most ironic terms. ''Someone in a Tree'' is a charmingly descriptive account as seen from differing perspectives. ''Please Hello'' consists of an amusing pastiche of various Western musical modes. And ''A Bowler Hat'' contemplates the long-term effects on a culture of assimilating dubious new ways and values.

Notwithstanding a tendency to campiness, the revival staged by Fran Soeder is true after its fashion to this innovative work. It is admirably served by Ernest Abuba as the Reciter (and in several incidental roles), Kevin Gray as a samurai who trades his warrior tradition for a bowler hat, and John Caleb as a fisherman who takes up the samurai sword when he sees tradition yielding to demoralizing westernization.

A musical of much more modest pretensions has opened on lower Fifth Avenue. ''Feathertop'' is the latest work of the WPA Theatre, the people who gave us the still-running ''Little Shop of Horrors.'' From the little shop's carnivorous plant life, the WPA turns to 18th-century New England witchcraft. Bruce Peyton (book) and Skip Kennon (music and lyrics) have expanded - some might say inflated - Nathaniel Hawthorne's slender tale about an avenging witch and her scarecrow-turned-human being into a chamber operetta with plot amendments and ramifications.

A pleasantly singable score receives an enthusiastic performance from a cast directed by Susan H. Schulman. Stephen Bogardus achieves a dashing style as the Scarecrow/Lord Feathertop. Laura Dean (the Boopsie of last season's ''Doonesbury'') makes an appealing heroine of the temporarily smitten Polly.

While Off Broadway's catalog of songs may not be as long as Nanki-Poo's in ''The Mikado,'' its range is considerable. A representative listing would include revues like ''A ... My Name is Alice,'' ''Rap Master Ronnie,'' ''Shades of Harlem,'' and ''Forbidden Broadway''; revivals like Puccini's ''La Boheme'' (starring Linda Ronstadt), Romberg's ''The New Moon,'' and Styne, Comden, and Green's ''Bells Are Ringing''; ethnic works like ''Kuni-Leml,'' ''Mama I Want to Sing,'' and ''The Golden Land''; a popular miscellany like Geraldine Fitzgerald's ''Streetsongs''; and, of course, the longest runner of them all, ''The Fantasticks,'' now celebrating its 25th anniversary year.

Such are some of the sounds of music Off Broadway.