I get the same problem and my Volvo dealer change speaker (reaer left) quick
and easy.
You have to take belts out of holster, after that you have to pull whole
thing UP and BACK at an angle of about 15degrees (bothsides -two people).
Take it in midle and gently pull up and to You. There are no screws or
anything else fixing it! it just fits there!
Speaker is very "Big" and it costs at Volvo Austria about 100 EUR (HU 601),
for that price is not possible to get another that quality.
Even I could change it by myself if I knew how As You now, it was worth to
live it to Volvo, for screws You will need flexible screwdriver there is not
enough place for ´normal screw driver.
Volvo speaker has also special plastic ring which hold whole construction
and it would be not nice job to repalce with another kind of speaker.
Next if You change speaker You need to exchange both, at Volvo you can pay
just for this bad one.
At my opinion go to service purchase original speaker an live them to show
how to exchange o even to do it, it is easy quick and cheeper than any
selfmade solution.