How are sound waves and gravitational waves different? What is the difference between sound waves and gravitational waves?
The source term in GR as I understand it is the energy-momentum tensor which contains pressure, which makes me think sound is a subset of gravitational waves. Are there more source terms?
 A: Sound waves are different from gravitational waves in GR in many ways. Sound waves are disturbances which propagate through some material medium, while gravitational waves are understood as a disturbance in spacetime itself, so they need no such thing. Sound waves also travel at some speed defined by the properties of the medium, and gravitational waves travel at the maximum possible speed, the speed of light.
Pressure in sound waves and in the energy-momentum tensor mean different things. In sound waves, the pressure measures how much the molecules in a piece of the medium are pushing around on it's surroundings. The molecules in a medium are constantly moving and bumping with each other, so that a force at some point in the medium causes a change in pressure that propagates at the speed of sound.
In GR, we have a tensor that measures how much spacetime is curved, and this curvature is determined by the local distribution of mass-energy. The diagonal components are called pressure by analogy with material media, but they measure kinectic energy, which is correlated with pressure in an ideal gas.
Also, to create gravitational waves that aren't immeasurably small, you need really heavy bodies moving really fast. Although Einstein himself predicted their existence in 1916, only recently in 2015 scientists were able to detect them for the first time, by observing the ripples produced by two black holes merging about a billion light years away from us.
