I just tried a few experiments.

Wind zero, clear skies, at 6000 feet MSL, engine idle, trim full nose up.
With 20 gallons in the tanks and no payload (something that would be
impossible to try in real life), as I pull back on the yoke, the stall horn
sounds at 43 KIAS, just like the book says, and a stall begins as soon as the
speed falls any further. With full tanks, 480 lbs payload, and idle throttle,
the horn sounds at 50 KIAS, using the same method. At full throttle, same
full tanks and 480 lbs of payload, again carefully pulling the yoke back, the
horn won't sound until about 45 KIAS.

Does that sound right?