From any class that has a reference to the view which contains the buttons as subviews, you can just iterate through all of the subviews and see which ones are UIButtons:
for (id subview in _buttonView.subviews)
if ([subview isKindOfClass:[UIButton class]])
[(UIButton *)subview addTarget:self
selector:@selector:(playSound)
forControlEvents:UIControlEventTouchUpInside];