They've Chen... Chen is one of the main vocals and idk but Chen voice is already good for their songs. And I've noticed that DO goes always from K to M when they're performing but idk. But you get my point about the music shows etc because of that I think it's better for ot4 to sing their own songs. Even when Luhan was there Chen got more parts because he was the main vocal and Luhan was lead vocal, but DO is main vocal too so idk if they add DO to all CHI songs I think his voice will overpower more Chen's voice..