Have you considered hosting a casting call inside of a classroom?
Classrooms are spacious and affordable venues for special events. High schools, universities, museums, office buildings, and other spaces all offer classroom rentals. If the classroom is set inside of the building and there are few other events happening during your booking the space should have few distractions.
That said, if you want a pristine acoustic environment you'll want a space that's soundproofed or more private as a casting room. Theaters and auditoriums are an excellent choice. They offer acoustic environments on par with the final performance venue as well as a professional stage for actors to perform on. These venues also have amenities like changing rooms, makeup stations, costume rentals, stage curtains, and stage lights if needed. If you expect a large turnout for your casting call, consider a theater with a waiting room or lounge. This way people can go over their lines and wait in comfort without disturbing anyone currently auditioning.
Loft studios used for special events or artists are also great spaces for a casting call. You'll likely need a separate waiting area for actors who arrive early. But the main space of the loft provides a soundproof space that can easily be customized using studio backgrounds, lights, sound equipment, and other accessories. Lofts are also flexible when it comes to the rental terms, with rentals by the hour being the norm.