The recently updated Haswell-based 2013 MacBook Air is an incredibly popular line of laptops, but has unfortunately had its share of birthing pains. First, users complained of not being able to connect consistently with WiFi networks. Now Apple support boards are filling with complaints about volume settings that vary all over the map while watching videos.
According to a post on Macworld UK, other apps affected with the issue include Chrome and QuickTime. The post quotes one user who complained that volume fluctuated while watching videos, with another saying that the fluctuations only happen when volume is at a middle setting and are nonexistent at either high or low volume levels.
As with the WiFi issue, it's quite probable that the sound problems will be fixed soon through a patch in a future version of Mac OS X 10.8 or perhaps through a MacBook Air-specific firmware update.