Joined to share the solution to something that has been plaguing me since moving from Lion to ML last year.
I started of with a 2012 MBP 13" non-retina, and when using SIP (Telephone, Nimbuzz) randomly (every 10-60 seconds) the USB headset I was using would cut out for about 1 second, then return. When it did this, it would sound exactly the same as when you pull the headset out of the machine (there is normally a little white noise in the background when it is plugged in and active as an audio device).
I figured the headset must have had it, and got another one (different brand, had Mac compatibility on the box) and the exact same thing happened.
I then bought Blink and found the problem went away - assumed it was the apps not updating to a new API in ML.
Last week I upgraded to a Retina 15" MBP, and was on a call today and the issue returned - even in Blink. Tried Telephone - same thing.
I am running the latest ML 10.8.3 build.
This time, however, I noticed the Airplay icon was disappearing/reappearing whenever the sound cut out!
I unplugged my Apple TV - and now have perfect audio even in the Telephone App I was started out using last year.
Just wondering if anyone has come across this before, and I should point out that it NEVER effected music - just SIP calls - and was not silence detection or anything like that in the SIP side.
I am on gigabit fibre internet, 5 meters from an Airport Extreme (405Mbps Transmit Rate) - the Apple TV is plugged into the LAN interface on the same.
Has anyone noticed their Airport Mirroring icon in ML disappearing from time to time, and has anyone noticed any effect on the audio when it does this?