AND WAIT, THERE'S MORE: Amazing Grace Records, a new label specializing in contemporary Christian music, has a novel way of discouraging listeners from making unauthorized copies of the firm's records. Its first release, "The Praise Guitar," contains the warning label: "Wouldn't making copies of this record violate the eighth commandment?" (For those of you who have forgotten, the commandment's message is simple: "Thou shalt not steal.") . . . Catherine Mary Stewart (who you probably remember as the young Joan Collins in "Sins" and the co-star of "Mischief" and "Night of the Comet") makes her singing debut in "Scenes From the Gold Mine," a new Hemdale film which starts shooting here this week. The actress plays a (how'd we guess?) struggling rock star and performs a number of songs, including a new Bryan Adams tune ("Don't Let Him Know"). The film, due out in late fall, also features a pair of other new Adams songs, "Play to Win" and "No Way to Treat a Lady."