I had the same problem after a screen repair. I also replaced the ribbon and speaker and still no sound. It seems Apple devices do not like having things replaced. I had a similar problem on a 4S after a power ribbon replacement where the phone went into a permanent search mode. I tried all the "fixes" I could find on line and nothing worked.
It is an Apple "thing" and it doesn't happen on every phone which leads me to believe it is a software/ hardware issue.
I also think the same thing happens to the Apple repair technicians and we just don't hear about it because they have all the resources to fix the issues that come up during these repairs.
I never solved or found a fix to either problem.