buzzer doesn't work at testing with DC power
They may get damaged if you continue to power them with DC. Buzzers work in similar fashion with speakers but they can't reproduce a wide spectrum of audio frequencies.

Being used in the PC increases the possibility that the buzzer was fed with a square wave audio signal of a few (1 - 2 ) kHz.

I suggest you build a 555 square wave generator for your buzzers. Something similar to this,  that should drive a transistor with the buzzer as load.

 

Circuit source: http://ehelion.net/projects/digitalclock/555timer.html