Have you updated to the latest firmware version WW-15.2016.1902.507 yet?
How to update firmware: https://www.asus.com/support/FAQ/1009748/
If the audio is still not working, please try a factory reset. On account of the data will be cleared after resetting, we would suggest you back up your data before factory resetting.
If after resetting the problem still exists, we would advise you to bring your phone to your local service center for a further examination.
Service Center: https://www.asus.com/support/Service-Center-All-Countries