I have a radio very similar to the one listed here:

http://www.ebay.com/itm/King-Hinners-Si ... 2569e81f28

I noticed that when I plug the speaker into the terminal on the left, the radio runs on all 5 tubes, but it only runs of 4 tubes when it is plugged into the middle. When I plug it in all the way on the right, only three tubes light up. What is the purpose of the 3 audio terminals? Is my radio operating correctly?

If you had a strong local and using headphones , you could get by with audio out of the detector , the whole idea is to conserve batery power , if you had to take the A battery into town to have it recharged that was a big deal ,and B and c battery where not cheep also , so by using three tube istead of 4 or 5 thats what you did , your and your wife might get close to the speaker and use 4 tubes .With the kids and or nieghbour all 5 would be needed for enough volume