No matter how good your speakers are, the cones are still vulnerable to damage. Speaker cones are made out of thin sheets of material, and that material is under tremendous stress. The driver transmits vibrations through the speaker cones, slowly wearing them out. Even a small rip can make an annoying buzzing sound that grates on the ears of serious audiophiles and casual listeners alike.
Unscrew and remove the screws holding the speaker grill in place. Look closely at the speaker and try to find the rip. Most speaker rips are quite small, so it may help to go over the speaker with a flashlight to get a really good look.
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If you can not find the rip, turn on the stereo with low volume. Look at the speaker to see if the paper is vibrating erratically or flapping at some particular part. Listen closely to the speaker to try to hear exactly where the buzzing sound is coming from. That is where your speaker is ripped. Once you find the rip, unplug the speaker.
Mix white glue one-to-one with water. Use a very small paint brush to apply a thin layer of glue around the rip. Place a small piece of tissue paper over the tear and press it down.
If you can easily remove the speaker cone, do so. Put a small patch of tissue paper on the back. If you can't get the speaker out, skip this step.
Let the glue dry, then glue another small patch on top of the speaker. Depending on the size of the rip, you will want to use two to four layers of tissue paper.
Reassemble the speaker and plug it in. The buzz should be gone.