Thanks for the question. Although Cobb sought reinforcements from the AOT, he failed to receive any serious assistance. As Alan mentioned, however, there were soldiers in Columbus who were home on furlough or who had been detailed. There were also soldiers who had been in the hospitals in the city that were urged to join the defense. Also, in the days leading up to the battle, Cobb ordered that all Confederate soldiers returning to their commands or otherwise passing through the city should be stopped and placed with one of the several companies being formed for the defense of the city. In addition, there were soldiers who escaped capture at Selma or who had been transferred up to Montgomery from the Mobile area. And, of course, soldiers who were in Alabama and had been called to the defense of Montgomery and later forwarded to Columbus. There is also another possibility. Even as late as April 18, there were still groups who did not know of the city's fall who were attempting to get to Columbus to aid in her defense.
Wilson's Raiders (4th Div) captured Columbus on the night of the 16th. On the 17th, the 1st and 2nd Divisions arrived at Columbus (with the exception of the detached parties) and moved through the city. On the evening of April 17 at around 5PM, the advance elements of Wilson's force began their march from the city. This movement continued until around mid-day on April 18, when the last of the 4th Div (first in, last out) left. It is possible that the musicians you mention were picked up along the roads to Macon along with others. The exact circumstances are almost impossible to tell, but is certainly quite plausible.
As for the prison situation, it appears that when Wilson's Raiders reached Macon that they corralled a substantial number of prisoners. These included men who were in Macon when it surrendered, who were captured at Columbus, and who were picked up along the road to Macon. The prisoners were held until April 25 or 26. I expect the term prison may be used rather loosely. More likely, they were confined in a yard or other area. This seems to be pattern during the rest of the campaign. For example, at Columbus the CS prisoners (which included the 1500 captured during the battle as well as prisoners from Selma and other points along the route) were held in the city stockade, also in the yard behind Col. Mott's house (which Wilson used as a HQ), and in the city's abandoned forts.
I hope this is of some assistance, and I am glad you enjoyed the book.

Charles Misulia
Columbus, Georgia 1865: The Last True Battle Of The Civil War
(University of Alabama Press: Tuscaloosa, 2010)