One sound is loud and another is quieter. what must be different about these two sound waves?

Loudness of the sound depends on the "Amplitude" as loudness is different for two waves, their Amplitude must be different

In short, Your Answer would be "Amplitude"

Hope this helps!

Spontaneity tell us the direction of the reaction, but not how fast it goes. gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. g = h - ts. and, finally the sign of delta g is always negative for a spontaneous reaction.