iphone 6 ios 8 siri only working with external mic!
do you have any tweaks or something that works with bluetooth, and or the mic?

to rule out jailbreak, turn off your phone and turn it back on while holding the volume up button, this disables all the jailbreak tweaks... like a safe mode, if siri works fine, then you know it has something to do with your JB if not? then it has something to do with your OS