I am having this issue as well. I have a iPhone 5 GSM Model MD634LL/A with iOS 8.1 Build (12B411). I have tried the following:
- Reset all settings
- erase all contents and setup as new
- restore the device and setup as new
- DFU Restore the device and setup as new
- verified the mic is working with phone calls, Google app, dictation, voice control, and voice memos, no mic issues found.
- tried changing the voice settings from female to male
- tried changing the language used
- tried turning on and off Siri and restarting
- tried it on multiple networks
None of these steps have resolved the issue. This all seems to have started after trying to enable the "Hey Siri" feature, which I could not get to work.
The turning on of the "Hey Siri" feature may not be relevant but I have noticed a couple critical things. One, it is not a hardware issue on my phone. Two, it does work with an external Bluetooth device. (Since Siri is working with my Ford Fiesta connectin via Bluetooth). Even with Bluetooth turned off, and nothing configured, Siri is not even recognizing any audio input from the built in microphone. The issue started after the 8.0.2 update for me and was behaving as expected in the 8.0 and 8.0.1 builds for this model. Given it is working with dictation and voice control, which are driven by the same software mechanics as Siri the install seems to be successful for the sub system settings, but there must be some issue with the Siri App. I believe that at least for the issues that I can reproduce here on my phone is due to a faulty build and specifically with the Siri App not able to call on the internal Mic functions. As for a solution, I do not think there is any possibility for anything other than an update, which is why I am describing this issue in great detail so the issues can be addressed and resolved in a future update!