I find it (I have a jenday, not a sun, but they are variants of the same species, or subspecies depending on which classification you agree with, with near identical calls) not so much loud as high pitched. My mother had two cockatiels, and the male one would whistle non stop over and over again for 5 mins at a time. Now THAT I found unbearable. You might know the call I mean, having had a cockatiel yourself? I'd say the sun or jenday has a louder volume, but it is just the pitch that gets me. Like how a high pitched scream hurts the ears more then a lower pitched one of the same volume.
That said, he still does have outbursts, which is ok, I just ignore them. It is to be expected. Though, we had a problem where my mother (I'm visiting them for Christmas with Alex in tow) was paying him attention when he did it, and we were taken back to square one . Didn't take long to get him back into the habit, but it was frustrating to see 10 months hard work (ever since I got him home!) broken by one person doing what they were told not to!
You hit the nail on the head there! That's the reason I will probably never have an aratinga, it's not the volume it's the pitch. I can take a too scream, it's annoying but it's bearable, it's just that high pitched screechy whistle that most suns seem to do that hurts my ears and to me, it's easier to ignore a really loud annoying sound than it is to ignore a slightly loud, painful sound.
I can also relate to you on your problem with your mother. I have the same problem with my dog and ziggy. Our dog likes to be very mouthy with people and I have a lot of guy friends. I will spend months correcting him that chewing on people isn't okay and he will finally stop. Then all it takes is for one person to come over and start wrestling with him and it undoes everything I worked so hard at :/ And i have been working really hard with trust training ziggy and I have one room mate that will randomly come up and put his hands in her face and tease her, thinks it's funny, leaves and then I am stuck explaining why this bird is afraid of men :/
Your dog is called Ziggy? Thats funny, my parents have a dog of the same name with similar habits! He's a lovely dog, not mouthy, but he loves to 'spar'/play rough, and like you said, one person encouraging it is enough to do months of hard work! Drives them crazy!
(Edit, silly me, your bird is Ziggy! I read that wrong, by fault. I love that name though)
On conures though, agreed! I have becomed used to Alex's loud/high pitched calls now, but they still do drive me nuts at times though! (Like I said though, much work went into getting him to not use them! But he is naturally not as vocal as most, thankfully!) but I would probably not recommend an aratinga species to someone unless they could cope with the noise, and had steady housing arrangements. Mainly because of my first-hand experience with this, I'm in uni and we move regularly. Sometimes we have housemates, sometimes even sharing rooms! (But lucky me, I've always had my own room!) Most people do NOT want to share a house with a very loud bird. Even apartments or neighbours with shared walls can get driven nuts by the noise!
I could see someone in a similar situation to me with a more vocal bird (Like has been said, I've been VERY lucky with Alex and his vocalisations, he is quiet for a jenday!) having issues, even perhaps having to rehome a bird! I've opted for my own flat/apartment next year to avoid ever having these issues. Strangely, no one I have lived with has ever commented on the bird being loud, and I'm told he is 100% quiet when I am out, but it's always better safe then sorry!
Long story short, aratinga species are beautiful to look at (Some of the most beautiful IMHO) an if you're into macaws, they can be a good, smaller alternative (aratinga means mini macaw after all, and they can be quite similar! There are even cases of large conures and smaller macaws hybridising out there) and absolute cuddly clowns, but their are plenty of other species out there with similar personalities that make equally as wonderful companions, and usually without the potential for ear splitting racket! I've also found my jenday to be quite bitey, and very stubborn, but his sister (owned by the breeders) is an absolute gem and the most placid, loving creature, so I think it may just be him.

But, despite the loudness and the occasionally nippiness, I would not trade him for anything! I think that unless the OP likes loud noise, a GCC sounds great, and other birds with similar personalities and size (or so I'm told anyway) can be the caiques, which may be worth a look into too!