Here is the problem.
For a lot of people, when they update to iOS7, their voice microphone stops working. The speakerphone mic still works, but the voice mic quits working. This problem is especially bad on the iPhone 4.
Apple says this is a hardware issue. However, it seems a little too concidental that so many people (based on these forums and other sites) are immediately having microphone trouble after the software update.
On the other hand, perhaps the voice mics on a lot of phones (iphone 4 and other models) are simply going out. This would seem to be a hardware problem.
In my case, I feel like the software update was responsible for the mic going out. It worked perfectly until after the update. Then, mysteriously, the mic stopped working. Apple would say that this would just coincidence, but others have experienced the same thing.
Either way, the problem seems wide enough that Apple should consider a recall or free replacement in these situations. Either way, there is a big problem with the voice mic going out for a lot of people, and this affects voice call as well as Siri and numerous other functions.