When media refers to mass communications in general, or when it’s used synecdochically to refer to groups of journalists or TV news crews, it is often treated as a mass noun, taking a singular verb. For example, one might say “the media is outside” instead of “the media are outside.”
Some people insist that Latin plurals must be used properly even in English. If you wish to try to please these people—which is up to each of us to decide for ourselves—use media as the plural in all contexts unrelated to communicating with the dead. But don’t listen to those who say mediums is wrong. Medium comes from Latin, but it is now a long-established English word (with roughly 500 years in the language), and we can treat it as one. The plural media stays in English only by convention, not because it’s more correct.