Execs at Bobby McFerrin's record company are singing ''Don't Worry, Be Happy'' despite the tough sell they face for the Grammy-winning vocalist's latest album, Medicine Music. McFerrin, known for his jazzy, a cappella vocal acrobatics, presented EMI Records with a November release much less catchy than his previous record, the million-selling Simple Pleasures, released in 1988. That disc included the whistle-along ''Don't Worry, Be Happy,'' a left-field smash that led to McFerrin's startling domination of the 1989 Grammy awards. But for one of the most sales-hungry of all major labels, Medicine Music's December peak at No. 146 has been, well, worrisome. ''He had that one fluke track,'' says an EMI executive, ''but whoever dealt with him in the past realizes his core audience is only 300,000 to 400,000 jazz freaks.'' EMI promotion VP Jack Satter says the label expected a slow build for the new disc. But he calls it ''brilliant,'' and thinks at least one of its tracks might work well on radio. But, he says, since radio response has been disappointing, the company has to generate ''a huge buzz and huge sales'' first. That's why EMI plans to promote Medicine Music sharply in 1991, with press coverage built around McFerrin's winter concert dates and (or so the company hopes) video exposure on VH-1. This need for publicity also explains one of EMI's weirder stunts: At a recent record-wholesaler convention in Florida, buyers for record stores were handed copies of Medicine Music by a bare-chested McFerrin look-alike, decked out as the singer appears on the album's cover, complete with hair ribbons and colorful war paint.