What is the medium in which sound travels inside a bird's body?

Bird sounds are made in the syrinx, but of course, the air sacs, passageways and the bird's lungs, also contribute to the production of sounds.

When birds start to sing, the 2 passageways between the lung and the syrinx, closes. Then, the sir is squeezed into the air sacs by the chest muscles. Squeezing the air sacs that surrounds the syrinx forces each of the two thin membranes in the syrinx into the 2 passageways between the lung and the syrinx. This process does not allow air to travel through. Afterwhich, muscles around the syrinx contract and tighten one membrane and pulls it out into the passageway (1 of the passageway). Air in that passageway can now move, which passes quickly over the membrane. That fast moving air causes a vibration, thus, sound is produced.