Only a few specialized audio dealers have attempted to pass off phony Sonys. The fake tapes were sold mostly through discount-type variety stores, and even these eagerly opportunistic establishments are now being brought to heel by the threat of criminal penalties. As things stand at the moment, audio fans can buy Sony tapes at bonafide audio shops with reasonable confidence. Should any quality problems develop, the dealer or the company will indemnify the buyer.

Sony isn't the only tape maker to have been ''targeted'' by fakers. Counterfeit TDK cassettes floated around the country for a while, and several other brands also attracted imitators in the past. Paradoxically, all these inferior fakes were sold - at full price -to people who prize quality. Nobody, after all, bothers to counterfeit so-called bargain tapes.

All of which once again points up the vital distinction between quality brands (such as BASF, TDK, Maxell, Fuji, Scotch and others) and off-brand bargain labels. The purchaser of off-brand tape sold cheaply at various discount emporia is apt to be buying a pig in a poke. There is no clue to the properties of those tapes, and very likely they would be no bargain at any price. Poorly made with overly lenient tolerances, such cassettes are prone to jamming and snagging. What's more, the magnetic material itself may not be suitable for recording music.

What the buyer often gets in such cassettes is reject video or computer tape recycled through the back door. Since such tape is designed for a signal quite different from those representing music, their response in the audio range is erratic as well as strident and distorted in loud passages. This matters little to casual users with cheap portable cassette machines, whose only requirement is that a tune be recognizable and recorded words can be understood. But this type of tape is not likely to bring joy to the musically demanding.

At worst, off-brand tapes may even damage a recorder. Improperly polished and poorly lubricated, they may grind down the finely lapped surfaces of the magnetic heads in the deck, and they may shed oxide particles to clog the head gaps and increase friction in the moving parts of the machine.