Thanks for the help. I'll make note of the need to rewind the tapes back and forth, since there are eight tapes to be recorded. As a test of the tape deck's current speed capability, a random sample of several 7-inch reels have been tested, and they all play at normal speed. So, it's the 3-inch reels.
As some additional background, my brother called to ask about the progress. During the conversation, he recalled that the hardware used to record the 3-inch reels was two identical battery-powered portable recorders, each with a mini-plug microphone. This was back in 1970-1971, and our Dad purchased them as a pair in Germany and sent one to our Grandfather, so that they could exchange audio letters. Apparently, the recorders nearly drained the D-size batteries each time a 3-inch tape was recorded, so my assumption about speed variation due to low power was spot-on. I'll try to find a fix that varies the speed of the Akai tape deck.
Since neither the problem nor the solution is with Audacity, I'll close this topic.
Thanks for the help. During the past day or so, out of necessity, I've learned a lot about Audacity. I've used it a lot to record cassettes and vinyl albums, and to capture streaming audio from the web. Great program.