I know that sound doesn’t travel in a vacuum, you can hear it muddled in water, and different temperatures of air transmit better. But which is the “best”. Let’s say you had a speaker and a microphone in an enclosed case filled with whatever you wanted, which would be the best medium to ensure the best sound transmission?
Probably solid hydrogen.
I would say degenerate matter in a neutron star is a better conductor of sound. It’s densely packed and doesn’t have to deal with pesky things like electromagnetism slowing down the sound wave.