My parents are from Beijing and Nanjing. They only speak Mandarin and don't have any other accents. Chinese was my first language, but like any other Chinese-American kid, I was forced to learn English as well and the Chinese was slowly pushed out of my head, which is why most Chinese kids go to Chinese school on Sundays. Try telling most of EXO-M that they can't speak Chinese, because most of them were born in China. I've been to Beijing several times, thanks, and I've heard Chinese music on the streets and in other places of China.
All the members in EXO-M can also speak several other languages because their careers demand it. Also, Chen is the main singer and he's not Chinese. That's why a lot of the words he speaks sound off, and I'm pretty sure they use his voice the most because it's the best. But the rest of them do speak Putong Hua, regular speak. My parents do as well. They spent the majority of their lives in China after all. Also, the auto-tune and the fact that the melodies were originally made the Korean language will make Chinese sound a bit awkward. I can tell you from the interviews with the four members that were native Mandarin speakers, though, that their normal speak is on par with what you'd come to expect from Chinese individuals their age.
And quite frankly, I don't see why you would want to say that their Mandarin isn't "proper". I don't exactly think that "proper" and your average Mandarin speaker's language can be in the same phrase together, as proper English and what is considered proper in many other languages are completely different from what is used in households. Proper regular speak? How does that work? Anyway, I can understand them fine and don't see why you would pick on them for that. Also, you questioned my parents of not speaking Putong Hua while you yourself are 1/4 Mandarin. Now unless your parents grew up in Mandarin-speaking cities and know how people actually speak in those parts of China and you can as well, I don't see how what you could have picked up from them is more valid as normal Mandarin language compared to my parents'.
lol both my parents speak Chinese all the time at home. I understand it pretty completely and can speak it to a lesser extent, though reading and writing Chinese was my weak point. I think it's the fact that Chinese has four accents, and when you're singing a song of several different tones that do not correspond with the accents, it doesn't sound quite right. There's your explanation.