So if sound money is such a good thing, what is stopping people from simply trading with each other in gold ?

The economy would stagnate due to lack of currency (aka specie) and the economy would stagnate due to the difficulty entailed in conducting a business transaction. How would you pay a bill through the mail? How would you pay for a tank of gas? Would we all roll around with bricks of gold in our cars? The reasons go on and on. A look back in history would show that people have traded with gold or silver in the past and they stopped doing it because it didn't work. An economy can have sound currency without having to trade in the actual gold or silver that backs the currency.