Under 'I/O Settings' choose the 'Stereo Mix' option.
Click on the '>' button next to 'Session Parameters'.
From the Quick Start menu, choose 'Create Blank Session'.
Double-check your input and output settings in the Mix window of Pro Tools to make sure that they're now set correctly and test your input/output again.
The input and output paths for your audio interface should now be set to the default settings.
Click on the Input tab and then the Default button again.
Click on the Default button towards the bottom left corner.
By default, you'll be on the Output tab.
Make sure your audio device (HDX or HD Native) is selected as the Playback Engine.
If you are unable to get input into or hear output from Pro Tools the following troubleshoots may fix the problem.