Which of these travels the fastest through a medium?

atom

photon

sound

electron

To answer this question, we have to identify the maximum speed that could be achieved by an atom, electron, sound, and photon.

Atoms don't have a fixed speed in a medium. The speed of an atom is determined by the temperature in the given medium. At absolute zero, an atom would be stationary.

Electrons can travel nearly as fast as light when supplied with immense energy. To make it simple, the faster an electron moves, the "heavier it gets". Electrons were able to move at 99.9999992% of the speed of light after being supplied with 4GeV. Electrons in electricity travel at 1% of the speed of light.

Sound travel fastest in non porous solids such as iron. However, it still wouldn't match up to the speed of a photon.

Photons travel at a constant speed: 300 million meters per second. It is the highest limit of speed achievable.

So to answer the question, photons undoubtedly travel the fastest in a medium.