Controlling DC (Pager) Motor with Audio Signal
Use PWM to drive the pager motor, at a frequency above human hearing (so you don't get audio whine from the PWM).

You'll need to choose  pager motor, and then characterize it to find what duty cycle produces what approximate frequency of spin.  Each motor will be slightly different, but motors manufactured at the same time should be close.

You won't be able to get an exact frequency without some form of feedback.  If you can get ahold of 3 phase pager motors then you can set the exact frequency of vibration, but they aren't easy to find, and are harder to drive properly.  They are the same type of motor used to run hard drive platters and CD/DVD discs.  If you rip apart a CD burner you should be able to take this motor, and you may be able to hack the motor drive circuitry to do what you need - but then you have to attach your own eccentric weight to the motor, and the whole assembly is going to be much larger than a pager motor.

But if you're just looking for a close approximation, you may be able to characterize a particular pager motor and be satisfied with that.