I had the same problem last week too. I went apple store, they did restore, and then changed speaker module. And then I came back from the store, and didn't do restore from the backup at that time. On the way back to my home, I got 3 missed calls. OMG! I did go to the same apple store, and they swapped with new 16GB iPhone 5.
When I arrive home, I did restore from the previous backup, and it worked.
I also got the same problem last night, my iPhone 5 speaker is not working again, I missed several calls because of this problem. I went the same apple store. I told the guy that I got the same speaker problem, and he asked me try to restore. I said, I did, but didn't work. I told him I had the same history last week.
Finally, he told me he changed speaker again. After changing the speaker, it works again. I asked him what's the exact problem, hardware or software? He said it might me hardware problem. Then, he told me don't restore my phone from the previous back, and to use as a new phone. I cannot accept his idea because I have some my personal photos, data. He said that they cannot swap with new phone so many times if the speaker problem happens again. I have no idea why he can say their customer like this. This is not totally my fault. I asked him why I cannot restore from backup? He said because the problem happened because of that backup? I asked him are you sure? He said NO.
I only told him I don't want to come apple store again. I need to do my business. But he told me try to use without restoring backup, and cannot swap several times.
The phone is only 1 week old, and I also have apple care plus.
So, Apple don't try to release next generation/ new iPhone, and please fix this kind of problem first to keep your truly apple customers.
If not, a lot of apple customers will be gone soon.
You know there are a lot of choice to choose for so many reasons in stead of apple nowadays.
By the way, here is your Employee (186267669) who made me to be unsatisfied & mad. 😠