The means of measuring that audio level is with a multimeter; most work fine past 20KHz (AC) and will give you an average reading in audio Volts. Can be half a volt, 1 volt, 4 Volts full blast... just measure, won't bite you because it's audio instead of 60Hz AC.
Try your RadioShack relay; at your home stereo first, measuring the voltage at which behaves the way you want. If your cell phone audio can sustain that same level with your relay connected, well, done.
Another way is to use a transistor (Darlington?) ; saturating it when audio peaks feed the base and will pass DC at that audio 'pace' . As the load is a motor, should work fine, the motor won't start/stop in a millisecond, keeps spinning by momentum until real low signal (silence) cannot sustain transistor conduction. Try this, perhaps works without refinement:
Cao--------1KΩ-------B
+4V---------M--------C
Gnd-------------------E
Cao=Cellphone audio out
B=base
C=collector
E=emitter
M=your tiny motor
+4V and Gnd = your 4V battery
Go play with it!
Miguel