The explanations that Jameswei gives above are correct. After the aluminum wire loosens then it can begin to arc to whichever connection is has come loose from, or even to an adjacent piece of metal. I own a remodeling/home theater business in Boulder, and we have found outlet boxes that are completely black on the inside from arcing over the years. Very dangerous. Aluminum wires should have never been allowed for 110V internal wiring in houses.
A safe, code-approved way to correct the problem is to buy Ideal brand purple wire nuts. Most Home Depots or electrical supply houses carry them. Wire nuts are the rounded-cone plastic pieces, threaded on the inside, that you will see twisted around the ends of electric wires inside of a wall box, where one or more strands of wire are twisted together; the wire nut basically keeping the bare part of the wires squeezed together and isolated from other wires or other metal. All wire nuts are color-coded for size of wire(s) to be inserted. The purple wire nuts have a gooey substance inside that prevents oxidation between the aluminum and copper wires when they are twisted together. A small length of copper wire is twisted onto the end of the aluminum wire, then that twist is inserted into the purple wire nut. The other end of the copper wire is then connected to the outlet or switch/dimmer, etc. You can do this yourself, although I would not recommend it unless you are experienced/ comfortable with doing electrical work. When we do an older home remodel we do the above operation on anything we touch because of the liability issues, and quite often the homeowner pays extra to change out most or all of the other outlets/switches in the house. Licensed electricians usually charge $18-$25 for each device/outlet, incl. the wire nuts. If you have one come out just to change out a few outlets they may want to charge more to cover trip fees, etc.
That would be a great justification for having them run some dedicated 20 amp circuits for your gear outlets while they are there. We now insist on 3 or more such circuits on every theater that we build. If you want more info on that, there have been a lot of posts on that subject here on Audiogon over the last few years.