From the start of the mission, you'll fall down into a room below. Go forward out of the room with Sonic, down to the pathway ahead, and loop around to the next platform on the other side of the 360 ...

The broken bridge serves as a ramp now, leading up to a few floating enemies. Use homing attack to go across, then left, to grab the floating sun medal. The second one is a bit easier to get to—just ...