Well I don't have a Volvo but here's what I have observed over the years:
a lot of those dual inlet rectangular muffs are made out of rubber too stiff to conform to the shape of the lower unit. I had trouble with some of them getting the pump in my Cobra to prime, which is at the end of the upper gear housing, so not all the way up on the engine like Volvo's or a Bravo. I tried the Merc/Quicksilver muffs with the metal rod and clamp and to me they are superior. They are soft enough to flex and the clamp holds it in place securely. Since I started using them, never a problem getting the Cobra to prime and pump water.
Also, this may work well for a Volvo or Bravo....what I do for the first start of the season, is install the muffs first. Then disconnect the raw water intake hose (Cobra's is on the stat housing, unless it's late model one just like the Volvo) and hold it up. Fill it with water with some Dawn dish soap mixed it. This will fill the hose and outdrive water cavities and the Dawn dish soap will lube your long dormant impeller. Then re-fit the hose and turn the water for the muffs on.
I do this at each spring start up and it works like a charm.