If anyone has a couple minutes to spare, could you try the following?
you will need this mp3 file: https://drive.google.com/file/d/0B3k...it?usp=sharing
. it is a 2 second long, 1 kHz tone at -3 dB encoded as standard mp3. A simple beep.
disable touch sounds on the phone. the tick tick that it does when you tap on the screen. we don't want inteference with other sound sources.
set that beep as ringtone and notification tone on the phone.
set music volume to just above 0 on phone. the lowest possible without muting it
start playing some music on phone
go to volume settings, the window with the 3-4 sliders.
the experiment is: tap somewhere on the ring or notification volume sliders, wait 10 seconds at least, repeat
does it play the beep with a hiccup? it is quite short when you listen to it this way. if you have headphones on, the pop should be very loud through the headphones.