Whenever a fuel undergoes combustion (this takes place in the presence of oxygen), energy is released. Depending on the amount of energy, we may hear some sound. Kindly note that both the fuel and the oxygen are required for fuel burning and energy release. Many times, especially in a classroom, a "pop test" is use to determine the amount of energy generated from combustion of a given fuel. An example is the testing of a hydrogen-oxygen mixture for the flight of a small rocket. In this test, the fuel is burnt in the presence of oxygen, and the loudness of the resultant energy release is measured.
Interestingly, pure hydrogen and pure oxygen will not generate any popping sound or will fail the pop test. This is because pure hydrogen is fuel, but cannot burn without oxygen and hence will create no popping sound. Pure oxygen, on the other hand, can help fuel burn, but is not a fuel by itself. Hence pure oxygen will not generate any popping sound.
It is a mixture of hydrogen and oxygen that will generate a popping sound and hence pass the pop test.
Hope this helps.