First of all does the computer speaker need DC or AC? If you switch your meter to highest resistance range and try the jack with the red lead to the centre and black to the ring, note the reading. Switch the leads and repeat. If the readings are the same then it requires AC, if they are different, then DC.
A pair of computer speakers would be rated at 5W each, so combined, 10W, allowing for inefficiencies the amp will be using, say 15 Watts.
Now for the voltage, try low(5V), there won't be any damage but the output could be low and distorted. It is most likely that the voltage will be in the region of 12-19V, if the loudspeakers are of high power , say 20W/ channel then the voltage may be higher (20 - 50V). One way of having an educated guess is to open up the loudspeaker where the power is fed to. In it on the PCB, there will be some little can components (normally with a blue or black sleeve but with an aluminium centre which the plastic sleeve does not cover. These will have their value and voltage rating on them, such as 100MF 16VW, 100 MF is its value, 16V is its voltage working rating, so if the highest voltage rating you can find is 16V, then its certain that the amplifiers operating voltage is lower then 16V, most likely 12V. So in this case, 15 W at 12V = 15/12 Amps or 1.25 A 12V , AC or DC as per para. 1
Frank