I'm just an ME & licensed electrician, so I will defer to more detailed answers from real EEs.
These are both AC motors. I'm not sure about small split phase AC motors, but I know that on industrial squirrel cage AC motors, reducing the voltage will not affect the speed, the motor will just have no power and stall.
The only way to vary the speed on an AC motor is by varying the frequency (which typically is not cheap). Still, you might find a bargain on eBay or PLCcenter or similar - especially in this small a size. You are looking for a VFD, Variable Frequency Drive, adjustable-frequency drive, variable-speed drive, AC drive, micro drive, or inverter drive (all synonyms).
With the current interest in all things energy conservation related, the HVAC industry (Dayton) may have introduced small inexpensive VFDs for fan motors just like the ones you are looking for. Motor and drive need to be compatible.