Remember your basic school physics. A changing magnetic field will induce electical currents in a conductor. Thus, placing one near a loud speaker will induce currents in the alumininium layer of the disk. This will cause it to heat up. How much will depend on the power of the loud speaker and how close to the coil the disk is. Placing it on a bar magnet will have no effect.
What you must remember is all optical media (CD's/DVD's) consist of a layer of aluminium that is only a few molecules thick + various optical dyes layed down on a plastic base and sealed with resin. The resin coat is also very very thin.
Heat can warp the disc. The laser then won't focus on the tracks and the disk will be unreadable.
The ultra violet light in sunlight will eventually bleach the dyes - making the disk unreadable.
If there is the slightest pit or scratch in the resin coat, oxygen from the air will eventually get in and oxidize the alumininium - making the disk unreadable.
Human sweat will eventually corrode the resin coating. Allowing oxygen to oxide the aluminium - making the disk unreadable.
The processes of decay are temperature dependant. The hotter it is, the quicker the decay goes.
Any combination of the above factors will ultimately destroy an optical disk. The trick to a long life is keep them clean, cool and in the dark.
Better quality disks probably have a better resin coat - hence they last longer.
