When the phone is connected via Bluetooth to any Bluetooth Device and the Bluetooth Device is turn off no audio or sound can be heard from the phone, the phone doesn't change to the speaker automatically and I have to turn off the Bluetooth on my phone in order to get the sound from the phone speaker. It happens often. I have installed the latest software update available and the phone is not rooted.
This is my exact issue as well! It sounds like we are getting to the point that we know what is causing this speakerphone/bluetooth issue. Now we need asus tech to actually take it seriously and get a patch to sort it ASAP. Make sure you let asus know (you can do this through the phone). The more people complaining the more likely this will be to get fixed.