First check that both devices supply or require a DC supply. (Or, of course, that they're both AC). If the terms 'AC' or 'DC' aren't on the labels, look for a 'tilde' ( ~ ))squiggle (which means AC) or a dashed line above a solid one (which means DC).
If both devices are DC, check that the tip polarity is correct. (There should be a little symbol on each device, illustrating the plug and showing whether the centre of the tip is positive or negative. They need to be the same).
Then look for the current rating on each device. (e.g. 50mA). If the figure on the power supply is the same as, or greater than, that on the speakers, you can safely use the power supply.
Chris