I wouldn't waste their time by calling the non-emergency number. Call the Department of Code Compliance (703-324-1300) if you really would like to speak with someone who knows more about the subject.
Radios, TVs, musical instruments, loudspeakers, or any other sound amplification devices may not be used between 11 p.m. and 7 a.m. if they can be heard:
Across property boundaries
Through common walls within a building
At 50 feet from a car parked on a public right-of-way or in a public place
For more information, call the Department of Code Compliance, 703-324-1300, TTY 711, or read the county’s Noise Ordinance
If you make sure that everyone within an audible distance is okay with it, and won't call the cops on you, there aren't any concerned citizens that will be driving by and call the cops, and the cops don't routinely patrol the area, you'll be fine. Just play it at your desired volume, and then if the cops come just turn it down. No need for it to be so loud, anyway. Louder isn't better.