When the cones are weak they let the coil on the bottom rub against the magnet. The only solution would be to replace the speakers.
The other possibility is the impedance is incorrect from the device you're feeding into it. In other words let's say you took wire from output that is supposed to go into the a speaker and plug it into the mic jack. This signal is already amplified and went sent through the amplifier in the PC it will be completely distorted.
There are ways to correct this but involves matching transformers. I'm sure you don't want to get into that.