I was having the exact same problem. When it's squealing it's actually slipping and not properly turning the alternator. When that happens you will see Volts go down to 12 because it is running off of your battery at that point.
I tightened the alternator/waterpump belt up and it went away completely.
I wouldn't replace the belt unless you can see on closer inspection that it is in need of replacment.
**** alternator belts........... make sure you get a high quality one that wont stretch too much....... NAPA makes a good belt. There are 2 sizes you can use for it. One is just small enough so that the tensioner has to be all the way out for it to be tight, and the other you have to bring the tensioner all the way in and squeeze it on........ atleast that was my experience....
It does not have a whole lot to do with the fan belt being loose. It has mainly to do with, that alternator is about ready to go out or voltage out put is too low. There is such a load on the alternator that it is tryiing to supply voltage to all accessories and charge battery at the same time from starting the car. Try a higher amp alternator and see if the problem goes away.