I guess the question at this point is, what other activity is in the area? I would suggest getting a n EMF reading, but that would be useless near a microwave. They're like electromagnetic generators.
However, I would be curious what kind of activity you would hear on an EVP recorder. Is there other similar activity, was this an isolated incident? Any background?
If it was an isolated incident which has no other similar activity, then I would start looking for a physical explanation. With other supporting activity, then I would possibly look in all directions.