That would explain the Sound of Silence!
You'll find 5.25"/13cm speakers are a direct fit but you may need speaker rings to mount them.
https://www.ebay.co.uk/itm/VOLVO-740...5/290388292676
You'll notice two sets of holes, one set for the pegs in the speaker cover, the other 4 are for the speaker itself and those are different to the OE speaker lugs.
Also when you fit the speakers, you'll need to re-route that wiring that is currently through the speaker hole. That needs to be disconnected at the black plug, drop the loose end back through the speaker hole and bring it up through the lower hole, currently covered by the grey polythene. Similarly, you'll have to drop the speaker wires down from where they're held in place by the mastick so they drop down then bring them back up through the speaker hole - make sure both sets of wiring are tucked in tight to the door inner skin so they don't get caught in the window mech.
The amp will be either 20W or 25W RMS per channel, can't remember which so not particularly powerful by todays standards where most head units are 45-50W RMS per channel.
Don't get bamboozled by the "PMPO" figures on speakers - they mean nothing!
If they are the only figures quoted by the speaker manufacturer, divide by 4 to get the approximate RMS power handling figure.
When you get the speakers, consider that you might upgrade the head unit at some point so go for a higher handling figure than you think you'll need for now.
PS - might be worth checking the rear doors to see if they have speakers too!