The speakers are probably bout the only thing I hate on my macbook. They're way too quiet even when maxed out. It seems the spears are rear facing and are designed to bounce off the base of the laptop lid, which I guess for some people is fine, but horrible for me. As such I always wear headphones.
I guess if I wanted to blow 90$ I could get some bluetooth headphones
Far as the speakers. Voltage is only half the story, its possible to give the speakers a 6V power adapter, and still fry them if the amperage is much higher than the speakers can handle (or the speakers wouldn't work if the amperage range is too low for the speakers).
Kinda like hooking up a camera charge with the same voltage to a cellphone, but digress if the charger happens to use 2.2mA and the phone was only expecting say 0.7.
But basically you'll see The voltage followed by an equal type of sign, then the amperage typically. The same should be on the speaker next to the input. (The equal like sign show the type of current flow, or sometimes the polarity) .