Sorry I could not reply earlier but I had a long haul job on today.
If I was told to spray WD40 onto my fan belts I would think it was a joke. BD is correct to a point with his synopsis but not with the method of determining the fault. Now if BD had have said spray water on the fan belts I would have agreed, if the sound had stopped - usually very quickly, say 30 seconds the sound will re-apear then it could be the belts drying out is indicative of a worn perished rubber belt in need of replacement,or failure may be imminent.
Unfortunately a squeaky belt can not only mean a worn out belt it can also mean that the alignment of the pulleys may have changed slightly, due to wear and tear or even a nut or a pulley coming loose so it really needs a good visual check to determine the reason for the squeak. (Best done with the engine OFF)
Remember we are not only talking about just the two belts driving the alternator and water pump but the other belts concerned with driving the vacuum pump for the brakes and power steering.
I suppose there is even a chance the squeak could eminate from the timing belt.
So as you can see my thoughts are 'why contaminate the drive belts with a spray lubricant when we are told to keep all drive belts free of oils and contaminants, yes it will cure the squeak but it will not cure the problem if it hides the squeak for a period of time. Water will do exactly the same but safer and will dry out very quickly without contaminating the belts.'
I hope you find these remarks helpful and it helps you rectify your squeaky belts.
There is a fact sheet for changing the belts, you may need it.
Off to bed now, night.