Final answer:
The evocative sound that signals a new call for laborers, especially among enslaved African Americans, was drum beats, produced through practices like 'patting juba'.
Step-by-step explanation:
The evocative sound signals that call for laborers have varied across different cultures and periods. In the context of enslaved African Americans, drum beats are particularly significant. They used hollers or calls derived from their musical tradition of call and response, which were modeled on African drumming and reproduced through 'patting juba', involving rhythmic foot tapping, hand clapping, and thigh slapping. This specific type of sound served as an aural signal to bring workers from the fields, summon them to work, or simply communicate messages. These calls were fundamental in their communication, being one of the few means of expression and coordination available to them within the constraints of their situation.