The Federal Communications Commission has okayed a new system for broadcasting stereo on AM. Initial reaction among the audio-minded ranges from "Oh boy" to "So what?"

Listeners who might welcome the decision would be largely those who live in parts of the country where FM broadcasting is sparse, either in terms of stations or in the kind of material furnished by existing FM stations. Stereo AM is not going to change the technical limitations of mono AM (limited band width and susceptibility to noise and static), but at least it will provide a measure of the stereo experience and could act as an incentive to buying some sort of stereo receiving equipment.

Oddly enough, stereo AM also is seen as a shot in the arm for some major big-city AM stations whose ratings have slipped in recent years in favor competing stereo FM stations. The latter have taken over a large share of the rock audience, which the AM outlets hope to regain. (If this means a return by many FM stations to classical music, no one is saying so -- at least not yet.)

But the main audience for stereo AM is expected to be the car-radio listener for whom stereo FM is not always a reliable program source because of reception problems and distances involved while on the road. The audible difference between a car stereo tape player and the car's FM faciltiy often is quite disappointing. Stereo AM may encourage more radio listening in vehicles.

No one I questioned can say just when the first stereo AM sets will appear, although many firms admit having prototypes "almost ready" for car installation, and a lesser number claim to be working on home sets too. The general view is that a regular stereo FM/mono AM receiver can be updated to stereo AM by adding a "chip" to the circuitry. The added cost to the consumer could turn out to be $35 to $50. There is at this early stage no indication as to whether the manufacturers will encourage present set owners to bring them in for the conversion to stereo AM, or will instead try to sell new sets that have the stereo AM option built in.

The "So what?" attitude comes, of course, from confirmed hi-fi enthusiasts, especially those who are fortunate enought to live within good reception range of at least a few stereo FM stations that broadcast the kind of music they enjoy. This audience is aware of the audio limits of AM and, after exposure to wide-range clean stereo on discs and tapes, they are not likely to go overboard for the blandishments of two-channel sound on an admittedly inferior medium. At that, there are a few AM stations that do transmit a reasonably high-quality signal, and perhaps the stereo enhancement will encourage others to do the same. As of now, however, AM radio -- for the most part (and this includes, surprisingly, the AM sections of otherwise hi-fi receivers) -- remains at the botton of the roster of preferred program sources for the quality-minded listener.