The growing availability of cheap music—from free videos and streams to $10-a-month unlimited subscription plans—is sapping demand for digital downloads at the world’s biggest seller of music, Apple Inc.

Music sales at Apple’s iTunes Store have fallen 13% to 14% world-wide since the start of the year, according to people familiar with the matter. The decline is stark compared with a much shallower dip last year. Global revenue from downloads fell 2.1% in 2013, according to the International Federation of the Phonographic Industry.

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