I want to get the new universal dock, problem is that I'm not sure that the iPhone is able to play audio from its own speakers without any line-out from the dock. Is this possible?
yes, it will play audio from its own speaker in the universal dock. I use a universal dock with my iPhone 4 currently and listen to pandora while its docked.
as a further explanation, iirc as long as you dont connect a cable into the line-out port on the dock, the iphone will automatically default output sound through its built-in speaker.