As Wayne said, aftermarket pads are notorious for noise on Volvos. Whether using OEM or aftermarket, not only should you always use anti-seize on the pads contact surfaces (sliding edges, top and bottom) but Volvo makes a silicone based anti-squeal paste that's applied to the backsides of the brake pads (not the rotor sides) and most dealers should have and be able to sell you a small tube of it. If you need a PN, e-mail me and I'll post it tomorrow. We use those two compounds on all brake jobs with OEM parts and never have comebacks as a result, the brakes stay quiet.

A thread a while back dealt with this subject. One of the posters indicated that no matter what he did, lubricant, shims, etc. the rears squeaked with aftermarket pads. At that time the recommendation was to go with OEM rears and aftermarket fronts. Because the rears do so little work on these cars they hardly dust at all. I would try the anti-squeal lube on the rears and see if that does anything.