Yes you can use less voltage (a smaller battery) but I would not recommend using any higher than 9v, unless you are experienced with doing this test.
I have used a 12v drill battery but anything higher than that and you risk ripping the cone from the surround due to over extention of the speaker.

When you connect the second wire to the battery, just tap the wire (2nd wire being connected) to the battery post for a second and listen to hear if the speaker makes a poping noise. If it does the speaker is good.

Dutch

Thanks to all for your help! I wish I checked back here more often... Yeah, I was wondering if there was like a gel or something to seal up the cone, but the weirdest thing happened to me today. I was driving, and I had a bunch of friends in the car, they were complaining about hearing only one channel, as I turned off the right channel to save the speakers. So they wanted me to prove that the whole right channel was fried, so I turned the volume up, turned the right channel up, AND it was fine! It seemed like a new car. I was shocked, and my friends laughed. Funny story

You got PWNED by your car...now that'* funny

I'd bet on that amp.

Not the first time that'* happened...

wont be the last