"Extramural" is not common in the US. This is the first time I've ever heard it. "Extracurricular" is very common.

It seems that 'extramural' can be used synonymously with 'extracurricular' (in that both can be studies undertaken in addition to, or outside of, ones study), however, extramural seems to be of British origin and has as primary meaning: "a course of study arranged for people who are not full-time members of a university or other educational establishment." https://en.oxforddictionaries.com/definition/extramural extramural = studies undertaken by people who are not fx; university students extracurricular = studies undertaken by fx; university students, that are in addition to their courses at university