Ear Speaker intermittent, no damage to flex cable or speaker

I'm seeing an issue with the iPhone 5 speaker and I'm wondering if anyone else out there has seen it and identified what's causing it. I had a customer who's ear speaker wasn't working. He had not had any other repairs prior to the problem. He took it to a different shop and they replaced the speaker. It worked for a while and then stopped working. He brought it to me and I tested it with a pre-tested ear speaker and it did not work. I examined the circuit board under the microscope and saw no damage. I then changed the front camera/ear speaker/sensor flex cable. It seemed to take care of the problem. We did about 30 minutes of testing (both with phone calls and using the voice memo app) and the ear speaker didn't fail during the testing. He called me about a week later to say that it had stopped working again.

I had a second customer a couple of days later who has had no previous repairs on her phone. Her ear speaker was intermittently not working. By the time she brought it in, it was working. I tested the speaker using a multi-meter and it had continuity. Again, I saw no damage to the circuit board or the flex cable. I swapped out the ear speaker and sent her on her way explaining that the issue may come back and to let me know so we can try something else.

I'm wondering if there is a circuit board level issue similar to the iPhone 4 GSM version. That model has a problem with the chip that handles audio for phone calls. Over time after a few drops the chip starts to break away from the board. When you test the microphones using voice memo and video recording they work fine, but on phone calls they do not. I'm wondering if this is the iPhone 5's version of a similar problem.

Anyone had any experience with this?