well thats not the FIST thing i would check lol. wouldn't he have other MAJOR issues if the alternator was going bad lol.
here is a good way to find the problem, using the process of elimination!!!! yeah!
-take your stereo out and find your left/right front/rear channel wires.-(im assuming your not running an amp since you didn't say you were, but more information on your setup would be helpful when trying to solve the problem.)
-make sure you still have power coming to the radio, and run a line directly from your stereo to your speakers, if they all work, then you know that somewhere in the factory wiring there was a problem.
-also, make sure that everything is still plugged into the stereo, lol, something might have just come lose over time
-if its a problem with the factory wiring, then that sucks because that is the most difficult thing to fix in a car, since the wiring is hidden and really difficult to get to. however this is very unlikely.
-you said you still have power coming to the radio, so its not the fuses.
-are you running an amp or something? that might have just blown or died, or stopped receiving power.
-its really unlikely that your speakers blew, they give VERY obvios signs of needing to be replaced before they stop working usually, lots of static noise, scratchy, music distortion.
anyways try the hard line method and let us know what you find out.