Vinyl "Soundsheets" ... . . . are bound in magazines, used for sales pitches by a growing number of firms. The flexible plastic sheets bear-recorded sound on one or both sides, weigh one-sixth of an ounce. They're made by Eva-Tone Soundsheets, Deerfield, 111. September Popular Mechanics will include a '"Sound-sheet insertion featuring comedians Bob and Ray advertising a Boise-Cascade Corp. 'product. Public Relations Quarterly recently used the '. thin-plastic film to supplement an article about government broadcasts with actual sounds from Voice of America, Radio Moscow and Radio Havana. Oldsmobile used 600,000 "Soundsheets" to introduce the front-wheel drive Toronado, and direct-seller Amway Corp., Ada, Mich., sends out monthly pep talks on vinyl to its salesmen. The sheets cost from four cents to 6.3 cents each.