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POP/ROCK Van Morrison: ''Inarticulate Speech of the Heart.'' (Warner Bros. 23802-1.) The mood evoked in Van Morrison's music becomes increasingly mystical and flowing - less jazzy and rhythmic than his previous albums - as he is unafraid to explore many styles. In this album he experiments with Celtic legend, using synthesizer instead of bagpipes and jazzing up folk rhythms to R&B: ''Celtic Swing,'' ''Connswater,'' and ''Irish Heartbeat.'' These tunes are lively and enjoyable, but the repetitive chorus of the title cut, twice played, made this listener drowsy. Mr. Morrison calls on Whitman, Kahil Gibran, Yeats, and other poets in ''Rave On, Jon Donne.'' There is a depth of ideas in this song that is a characteristic treasure of Morrison's - but not so abundant on this album. On the last cut of the flip side, ''September Night,'' his husky voice breathes as he draws it out like a saxophone. ''Higher Than the World,'' and ''River of Time ,'' carry the listener along on an upbeat note. Mr. Morrison should be commended for forever branching out in style, as well as for the depth of his lyrics. This album hints, however, that he may be minimizing the use of prose for the sake of musical exploration. I hope not.