House bands for open mics are nearly non-existent. You should be prepared to bring whatever you need and know if it plugs in to whatever they have available.

Sometimes there's a house 'guitar or piano' that you get to play, but don't expect any human accompaniment.

I agree with Mac - best to be an audience member first. See if the audience respects the performers with attention and so forth. See if they have a PA system that works with the gear you would be bringing.

-Scott