By Katie Marsal

Published: 06:20 PM EST

Viewers hoping to hear 5.1-channel Dolby audio from movies and TV shows on Apple's iTunes Store are being frustrated by a known bug that disables the surround feature in software.

Apple is now known to be aware of the problem, which simply limits any Dolby 5.1-capable purchase or rental down to a normal stereo mix. The problem affects both Mac OS X and Windows owners and doesn't appear to be limited to the particular type of audio output used; if surround is enabled, users often encounter silence. Apple TV owners don't appear to be affected by the same audio glitch.

A developer contacting AppleInsider reports receiving word from the company that the flaw is a "known issue" being investigated by Apple's software teams -- though the company won't say what kind of progress has been made or how soon users can expect a fix.

All the same, it's increasingly clear that the company has been relatively slow to respond. Customers have been complaining of the issue in Apple's support forums since September of last year, or virtually from the moment iTunes 8 was released to the public.

While the exact cause for the problem isn't apparent, speculation by those affected blames the problem on how the audio mix is encoded and then played back in any app that uses QuickTime as its framework, whether it's iTunes, Front Row, or the regular QuickTime Player itself. Even if encoded without Apple's tools, the files will often only work in other playback software.

"I've discovered that opening an encoded M4V movie file in [QuickTime] Pro and selecting the Audio Properties reveals that the channels are messed up," one affected user writes. "Default is 'L, C,R , LFE, RS, LS' ... changing this to 'L, R, C, LFE, LS, RS' and then saving the movie as a '.mov' file allows you to play the movie in QuickTime or iTunes with 5.1 just by selecting the audio channel 'AC3 Surround' ... however, without a way to select the audio channel in Front Row, we're still out of luck on that."

Publicly, Apple has never responded to the complaints and at most issues refunds to customers who have bought or rented iTunes video content expecting the more immersive sound. For those shoppers who've encountered the limitation first-hand, however, Apple's direct claim on its iTunes HDTV page of computer-based 5.1 support is seen as an outright false representation of iTunes' true feature set.

"The surround sound in fact does not work on a computer," one AppleInsider reader says bluntly.