Mackey's in Old Town Manassas has live music. You can call them for the schedule of who and when is playing each week.
Okra's, also in Old Town Manassas, has live music on the weekends. You can view their calendar online and they also have links to music samples of the bands they have there as well.
Closer to you near the Battlefield is Addy's in the Best Western Hotel. They have live music every Thursday, Friday and Saturday night.
City Tavern in Old Town Manassas doesn't have live music (usually) but they have a bar in the back and an outdoor patio in nice weather. You can view their calendar here:
Clark's Grill & Sports Emporium is another option. A little farther away than the rest, a few miles outside of Old Town Manassas.
If I think of any more, I'll add them to the list.