It is fairly common for the rear pads to be a cause of a squeaking noise, there are a few things to look at. You need to use only Volvo brake Pads and Rotors, otherwise you may not be able to fix the noise. If they are Volvo parts, then it is important that the person who performs the work cleans the caliper, caliper pins and anti-rattle clips and uses the Volvo high temp silicone grease to help prevent squeaking. There are some special shims for the rear pads that help to prevent noises as well and they need to be used.