EMB medium is indefined medium?

medium isn't a thing it is a word given to a person believed to have contact or supernatural powers over the dead and spirits

Dependence of the speed of sound on the properties of the medium: The speed of sound is variable and depends mainly on the temperature and the properties of the substance through of which the wave is...

Yes, it can. Medium has stages, as in, it's not specific. You can't say that there is only a minor, medium, and only bright stages, because there are different levels of such.

If you had done that thing then use was otherwise use is. It really depends upon the conversation. How you want to start it.