Macbook Air headphone buzzing noise

I glean from the official specs:

sound pressure level: 125dB

This can mean just about anything, but if my hunch is right in that what they are trying to say is that their sensitivity is 125dB/mW, then this is indeed a highly sensitive set of IEMs, and I'm in no way surprised that they hiss to some extent when plugged into your MacBook Air. While the MBA has pretty low noise levels as far as laptops go, it should still be more than high enough for the IE80s to pick up.

Just to be certain of course: See if the noise changes considerably by plugging and un-plugging the power supply. Also check how much noise you can hear through a more normal set of headphones (iBuds, PortaPros, something like that).

If the noise level is objectionably high only through the IE80s, even when running the Mac on battery power, your choices are essentially to either live with it, use other headphones, or buy a usb DAC/amplifier with a sufficiently low noise level. Something like those from Leckerton or JDS Labs, for example.