Back in 2010, Apple extended iTunes song previews from 30 seconds in length to 90 seconds on songs longer than 2 minutes and 30 seconds, to give users a longer listening period before making a purchase.
It appears that some songs have now reverted to 30 second play lengths rather than 90 seconds, with many popular songs offering previews that are only 30 seconds in length on both the mobile and desktop versions of iTunes.
For example, previews of all songs on Sia's new album are only 30 seconds despite all of the songs being longer than the 2 minutes and 30 seconds, a length that used to result in a 90 second preview. Many other popular albums are also demonstrating the same short 30 second previews. Coldplay's entire discography, for example, has only 30 second previews instead of the 90 seconds of preview time that was previously available.
According to one MacRumors forum user, some songs that are only available for 30 second previews were available in 90 second lengths just hours ago. User complaints on Twitter also suggest that the shift is recent.
As early as a few hours ago these were all 90 seconds! If you remember, Apple made a big deal about switching to extended previews, now a lot of songs are reverting back. Seeing more 30-second previews now than 90-second on most albums I've checked.
Though most songs at the current point in time seem to be limited to 30 second previews, there are some exceptions. Certain albums continue to offer 90 second previews, as do all of the songs listed on Apple's Top Charts. For example, Leela James' album "Fall for You" continues to offer 30 second previews, suggesting the shift may be limited to certain artists or labels.
Back when Apple first made the switch to 90 second song previews, it experienced some delays due to licensing agreements, and it is unclear if licensing has once again become an issue. It is unknown at this time if the shift to 30 second previews is a simple glitch or a more permanent change.
(Thanks, MichaelSD!)
Update July 11 6:37 AM: Apple has restored 90-second previews for some, but not all, tracks that had been affected.