whether to use a muffler
In many cases, the central vacuum unit is installed in the basement or garage so the sound wouldn't be an issue anyway. I use all of my central vacuum units without pipes, just connect a hose directly to them and some are definitely louder than others. In particular, the two motor units are quite loud without a muffler. On the other hand, units such as the Drainvac Viper are quiet enough, even without a muffler that you can still have a normal conversation while in the same room. I think that unit has sound blocking material in the canister which does the same thing as a muffler.
Mike