Hi there !! 
Recently JTBC release an extract of the drama Man To Man (맨투맨) (that I didn't start to watch yet) and I hear an ost.. THAT OST ! 
https://youtu.be/ws5MwFc08Og?t=1m33s But I don't find who are singing and the voices sound familiar.. anyone have an idea please ? People say Vromance..