So, basically, they're saying you're better off putting an old portable AM radio in your car, and just listening to that? That's a horrible answer. If the other vehicles have the problem, it's still a problem. Quantity doesn't make it acceptable. My XC90 has the problem (as well as tons of others, it's been back at the dealer for over 2 weeks, 51 days so far and counting), but, that rattle is annoying when it's happening. It detracts from the audio experience, after all, that's what marketed to us, is the experience (especially when you go with the Bowers and Wilkins). A door rattle shouldn't be something that's acceptable - either they need better tolerances, or, need to insert some padding between components that cause this.
Tell them to file a ticket or send it up the chain.
Have you tried going to another dealer? You may want to do that, and also send an e-mail off to Customer Care and have them start a ticket. If you have a recording you could send them, attach it (or post it on YouTube or some other site, and give them the information, or, even send them to this thread). Or, if the dealer still refuses, talk to Customer Care and let them know you would like them to reimburse you to have the problem fixed by an audio shop since the Volvo dealer is incapable of fixing this problem, which should be a warranty repair.