Like all hardware, it could be the speakers getting old. However, this is what I would do:

- check the area that the cables are and make sure there is no dust. Dust has a habit of building static electricity which is problematic for audiophiles.

- check the cables you are using for the speakers. Make sure they are not loose.

- check the sound card. Make sure that it isn't loose.

If all of that is good and it's still popping, then take the speakers and plug them into something else (like an MP3 player) and see if you still get the popping when you play something.

These are the first steps I would take. Popping usually means there is some static interference.