JD2015: I have not depended on SIRI but, for the heck of it I tried what works for me: I have a Bose QC35 II bluetooth headset which contains a microphone (my only source and I have used it once with my phone) and I often use it for listening on my computer. Using Bluetooth, I have it paired with my Mac mini 2. In Settings/Bluetooth I set the Bose... to "Connect". Then down to Setting/Sounds, I set it to "Input". At that time, just tapping on the headset with the mike and the response is shown on a bargraph in the settings display. Then I turned Siri on, called "Hey Siri" and got my response from the speaker in the M2. I thought I would have to use the combo F5/microphone key but it was not necessary.
I don't know if a microphone plugged into the back audio jack would work or not. Worth a try.
Good Luck.