Go under the house and in the attic and seal the over sized holes that were drilled to run the wires with expanding foam. While your up there or under there there's going to be lots of other air spaces needing filling. Check around all the plumbing areas. Lighting and fan fixtures.
If this is a really old house more then likly it was what's called ballon framing. The walls are open from basement to attic. There needs to be blocking installed at the bottom and top of every stud bay.