As far as I know, there are no standards dictating the rejection of conducted or radiated emissions *into* audio equipment. There are standards related to emissions *coming out of* audio equipment (eg. FCC Class B) and usually when you mitigate one you also mitigate the other.
When it comes to conducted emissions into the power supply however, specific measures need to be taken. It's not clear to me that audio equipment manufacturers automatically build such measures into their products.
It's quite possible adding equipment upstream like cables, receptacles, etc could make an audible difference if such equipment is engineered-for-purpose and the site suffers from such problems. However, adding such equipment to mitigate emissions can end up affecting dynamics so things are not as simple as everyone thinks they are.
