While not a frequent flyer, I did fly common airliners quite a lot, and each time the (very) low sound quality of onboard public adress system has always struck me. 

We all know technology makes it possible to have a "HiFi" sound quality almost everywhere, so **why** don't major manufacturers (Boeing and Airbus) provide better sound quality systems?

While this is of course not mandatory to make an airplane fly, it is related to passenger safety. Better sound quality would allow crew instructions to be better understood during the pre-takeoff briefing and in the event of an emergency.

I have flown with several airlines, and there are already enough other factors that make it difficult to understand what's being said on PA:

* On board noise: air conditioning, running engines, neighbors talking, kids crying, ...
* Strong or difficult english accent when either the crew or the passenger are not english-native speakers.
* Crew speaking really fast.

The airplane companies can't do much on the above items, but they can improve the technical aspects. Of course, I do understand that this would require increasing weight (loudspeakers are heavy...) thus reducing commercial load.

**Side question:** How do US/EU regulations handle this point? Do they have any specs on sound quality requirements?

*Note: I haven't tried recent planes such as A380, so maybe it has been improved.*