If you have the manual climate control, the blower motor wire colors are green (with a red stripe) and black. For the electronic climate control the colors are yellow (with a violet stripe) and blue.

Yes, the voltage will change with the fan speed setting. However, with the connector removed (or an open motor), and a manual system, full voltage (12V) will be at the green/red wire for all settings (black is ground). The electronic system turns the motor on at off at a fast rate (about 100 times a second) to control the speed, so voltage will depend on what speed is being set by the control unit.

If you have the manual system, try the blower switch at the highest setting. This position bypasses the resistor module. If only the High setting works, the problem is not the motor.

Also, be sure to check the fuse (#31, 25A).

There's only one plug that's physically plugged into the blower motor and w/o a wiring diagram handy, I don't know wire colors but at full selected fan speed, there should be 12 volts there if the rest of the circuit's OK and yes, less voltage at lower speeds. If MCC, there is an old fashioned type resistor in the housing, lower left of blower motor but if ECC, it's a larger unit with two plugs and several wires. The ECC blower resistor costs about $110 so you don't want to guess at that as it's a non-returnable electric part.
It being winter (at least here now) and being that you might need to drive the car with heat, if it's not obvious you might be best off by paying a dealer a little diagnostic time to get it repaired ASAP. You could order a wiring manual for your car (800-25-volvo or online) but that'll take days to get.