When building drywalls with suspended ceiling do I need wall height to actual ceiling?
If sound proofing is your primary concern then you should build the walls all the way up to the roof, past the suspended ceiling.  You should insulate the walls with sound-dampening insulation. If you are willing to spend more money and time, you should look into isolating channels that the drywall attaches to.  This prevents sound from being carried through the walls and studs (normally sound hits drywall and vibrations are carried through the studs to the other side.  The isolating channels prevent this).

I do not think a suspended ceiling will be able to hold the weight of the insulation.  As well, usually a big benefit of a suspended ceiling is easy access to the plenum space above for running wires, accessing hvac/plumbing/fire equiptment, etc.  If you have insulation on top, everytime you need to take a panel out, you'd also have to take out a lot of insulation. It would also be hard to get it back in properly after removing only a single tile.  The tiles themselves should help eliminate the echo.