A survey of Apple customers by US firm NPD has found that more than a quarter expressed “strong interest in a free cloud-based music option”, with many saying they would pay a subscription fee for such a service.NPD reckons that between seven and eight million US iTunes users would be prepared to pay $10 a month either for streaming music or access to their music libraries on multiple devices, while 13-15 million would go for a free service. “If the consumers who indicated strong interest in a paid subscription actually adopted one of those services at $10 per month, the market opportunity is close to $1 billion in the first year, which is roughly two-thirds the revenue garnered by the current pay-per-download model,” says NPD's Russ Crupnick.