I found the solution for this from another source, but thanks to anyone who looked into it.
In case anyone else has this problem, here's a solution:
Opus default file handling
The menu sections for media file handling:
Settings/Preferences - 'Double Click' tab
Uncheck 'Use internal sound player for WAV files'
Uncheck 'Use internal picture viewer for: unregistered file types/all recognised pictures'
Can also effect default actions on other file types with this section of Preferences.