Audio Output Errors in Existing Projects

I’m currently having some apparent audio output issues in Cubase 7.5
The audio for external programs works fine - new Cubase projects work fine too.
However, when I open an existing project nothing can be heard and everytime I go to change the audio output in an existing project, Cubase freezes up.

The build & configuration:
Yamaha MR816x - Being used as the in/out box

  • No drivers installed
    RME Hammerfall HDSP9652 - Being used as the audio device
  • Drivers installed

The above configuration worked perfectly fine.
However, when I installed the Yamaha drivers to change the internal settings that’s when I started having all of these audio related issues.

Any help would be greatly appreciated.