It's a twin-engine petrol-electric hybrid. BMW, Mercedes and Audi all offer old fashined petrol engines, but as far as I can work out none of them offer a hybrid to compete with the T8, especially none that can give you >100mpg and 400bhp at the same time.
Yeah, the people who are actually buying the cars might have more important priorities than the sound the engine makes. Even if they don't, reading just two reviews - by people who have driven the and do so for a living - is enough to contradict your statement:
The 4-cyl D5 (225bhp) is practically made for the D5 and pushes the XC90 along very nicely
Sound comfort is up there with the best, it's compared to the Mercedes ML and Range Rover Sport
Volvo's 4-cyl diesel rattles less than the 6-cyl diesels of "certain competitors" *cough*
If anything, the petrol is the one that needs a more interesting sound, and that's precisely what's being dialed in during these last few weeks before production starts
Would they fit it with an engine they knew wasn't going to be refined enough for the car, especially when they are so openly going after the Germans?