Apple Off-Key on iTunes

For its part, Apple spokeswoman Natalie Kerris says "the conclusion that iTunes sales are slowing is simply incorrect."

Apple does not provide any financial details related to its online music store, but says 1.5 billion songs have been sold in the store.

Kerris added that iTunes sales are 6% of all music sold in the U.S., making it the fourth largest music retailer.

Forrester also found that iTunes shoppers tend to make "small 'impulse' buys," with the median transaction hitting $2.97. One-third of total transactions hit $1.08 or less, and 75% of purchases cost $9.99 or less.

"Apple's iTunes proves that 99-cents micropayments for digital music can lead to substantial revenue; buyers spent an average of $35 at iTunes over the past year," Bernoff wrote. "With half of all transactions costing $3 or less, though, transaction fees threaten to make iTunes unprofitable."

Using data from Apple press releases and public statements by Steve Jobs, Forrester analyzed total global sales and found that "Apple has been steadily selling just 20 iTunes tracks for each iPod sold, suggesting that even at 99 cents, most customers still aren't sold on the value of digital music."

In an interview, Bernoff said that even though song sales are down, it's the number of songs purchased per iPod that should have the music industry, not Apple, concerned.