Clicking sound originating from Airport Express firmware 7.5.2 and later

Question

Hello,
I have submitted a bug report through Apple Developer Connection (I am a registered developer). By posting this here I want to get support for our case: Make Apple fix the Airport Express firmware.
Summary:
The Airport Express device has a bug which appeared first in version 7.5.2. It produces a "clicking" sound when using AirPlay to transport music from an iPhone or a Mac, when certain frequencies are being played. This is not depending on the attached amplifier or headphones, digital or analog output, network settings (network created by Airport Express or not), Mac or iPhone used, audio cable or power supply involved or due to bad quality of the sound file played. The problems are not there when audio is played directly from a Mac or an iPhone through headphones or optical out. We and my colleagues (developers and audophiles) have ruled out all the other options. The problem boils down to a change in the Airports firmware starting with 7.5.2. In 7.4.2 the clicking sound is not there.
Steps to Reproduce:
Play the following song via AirPlay on an Airport Express device having firmware 7.5.2 or higher. The Airport Express being used in our tests is the model produced from March 2008 – June 2012. Attach high fidelity headphones (like Bowers & Wilkins P5, which we used) to Airport Express. Alternatively use a very good amplifier/loundspeaker setup. We also reproduced the bug for example on an Onkyo A-9070 amplifier with both optical and analog connection - there is no difference there. The loudspeakers we used are passive Dynaudio X12.
Play track no. 6 from the "Songs without words" album: http://itunes.apple.com/de/album/lieder-ohne-worte-op.-67-no.-2/id209380163?i=20 9381206
Listen to this solo piano piece carefully for at least 40 seconds. You will notice a clicking sound with high frequencies. This clicking is *not* there in firmwares 7.4.2 and earlier.
Expected Results:
Perfect audio quality as in firmware 7.4.2 and earlier.
Actual Results:
Reproducible clicking sounds in high frequencies.
Regression:
This problem appeared first in Airport Express firmware 7.5.2 and has not been fixed yet. It can also be observed in the current version of Apple TV (as of September 16th).
Notes:
This issue, while not being recognized by the casual listener, makes AirPlay unusable for critical listening - at least when using any firmware after 7.4.2. I would love to use the current firmware (which produces a much more stable connection), but I (and other audiophile AIrplay users) would love to see this issue fixed first.
Here is a thread that concludes - as we did - that Airport firmware 7.5.2 and later is the source of the problem. It is not the hardware, neither Apple's, nor the connected devices:
https://discussions.apple.com/message/17296916#17296916
Hope this helps!
Best,
Gabriel

Answer 1

I'm having the exact same problem. As a classical music lover, this is driving me nuts. It's especially noticable in soft passages in the music, when the volume is turned up high. When the dynamics increase, the little pops/clicks appear (they sound a bit like static noise, as someone stated earlier in this thread). It happens at irregular intervals. When you play pop music, the clicks will disappear (drown) in the music. But for true audiophiles who listen mainly to jazz and classical music this is a severe problem.
I have checked everything and it's definitely an airplay issue. My apple lossless files are 100% okay and play without any problem, except when streamed via airplay. Also, it makes no difference whether you use the analogue or the optical ouptut from the airport express.
I've created a sine frequency sweep myself in Adobe Audition, from 20 Hz up to 20000 Hz with a duration of 25 seconds and a level of -6 dB. 
When played through airplay, the clicks always start to appear after 4 seconds in the sweep, only to disappear at 23 seconds.
The clicks manifest themselves at different volume levels. To my ears, they make the same sound as when you change the volume in iTunes when streaming music through airplay. Only when you set the volume in iTunes extremely low, the clicks will disappear.
Apple engineers should use a frequency sweep to recreate the problem, as this is very revealing.
PS: file format isn't an issue here. I've converted the Apple Lossless file to MP3, and it makes no difference at all.