@jamietuley my first question is if this a RetroPie build image or did this come from something else. Honestly, hissing is something I've never had an issue with with any of the 4 Pi3bs I own, nor was it a problem with my Pi2 before.

I don't know why you would have needed to alter any config file for the sound. It's better not to play around with any of the base configs and just use the menu to turn things on and off. You can try the Raspi-config menu and see if anything regarding sound is not active. But I think the Audio selection in the RetroPie setup does the same.

Anything above that is above my skill level. I've not experienced the issue you're having, nor did I have to play around with the config file to get anything to work. I used the menus only.

Sorry