A week after paying Apple for a refurbished 4S I ran into this "audio compiler" related defect.
It is described here: http://iansiphonerepair.blogspot.com/2012/05/microphone-issues-with-iphone-4.htm l
"If this audio compiler doesn't work, then the person on the other end will not hear you - or they will hear a very muffled voice, or a lot of static. Due to what is largely believed to be a manufacturing error in Apple's Chinese factories, a number of iPhone 4 models were assembled with this tiny chip soldered to the mainboard (motherboard) improperly. Either too much solder or glue was used, and the connection is tenuous at best.
The problem is that the phone could work perfectly for a very long time, and then after a drop (even one that doesn't break the glass), a hard jar, or even for no reason at all, this chip loses its connection to the mainboard and causes the problem you may have experienced."
I will return the phone to the Apple Store. Hope they believe me. It is erratic on this phone, but I'll try to catch it on video.
Assuming the manufacturing error was a cause Apple should do a recall.