THOUGH 1990 was the best year ever for sales in the recording industry, raking in $7.5 billion, 1991 may be a different story.Sales have slowed due to the recession, even though small-ticket items such as records, movies, and video-rental have long been considered invulnerable to sluggish economies. "We thought we were greatly recession-proof - until the first half of this year," says Tanya Blackwood of the Recording Industry Association of America in Washington. The association reports an 11 percent decrease in the number of CDs, cassettes, LPs, and other units sold by record stores in the first half of this year, compared to the first half of last year. "It's been a difficult year, there's no question about it," says James Gavigan, vice president of sales for Sony Classical. But "in the last week or so, the news has been more favorable." "Retailers [record stores] are reporting that sales are picking up, and they're fairly optimistic about it." Manufacturers are hoping the period between Thanksgiving and Christmas will give the industry a chance to recoup losses. "It's the highest selling period of the year," says Marilyn Egol, a spokeswoman for BMG Classics. With the slow economy, "people are staying home more and need music to lift their spirits." At the Tower Records store in Boston, the cost of popular and classical compact discs runs from $4 to $14. "It's still a great price point for last minute gifts or for people who aren't spending as much money as they used to," says Robert Stapleton, New England regional manager at Tower Records. As for the recession, he remains cautiously hopeful: "We could always be doing more business, but we're not as worried as car dealers and people selling the more high-end items." Despite record companies' reliance on staples of the past - such as Tchaikovsky's "Nutcracker," multitudinous versions of Handel's "Messiah," or Jessye Norman's "Christmastide a few brand-new Christmas selections have managed to emerge from the pack. A strong seller this year is a "crossover" Christmas album by the Chieftains called "The Bells of Dublin" (RCA Victor/BMG Classics), featuring pop artists Jackson Browne, Elvis Costello, Rickie Lee Jones, and other special guests. In the classical music category, "Christmas With Thomas Hampson" (See story at left) is also doing well at the cash register, Mr. Stapleton says. "We do try to pick out things that would be unique," says Ms. Egol of BMG Classics. "How many times can you buy 'Silent Night?