What are you referring to in the photo? The spot just below the fuse panel where the thin bitumen panel is pushed in is not a drain. There shouldn't ever be water there. The round plastic plug further down can be removed, and if you see any leaves inside the sill, vacuum them out. Then reinstall the plug. The sills have about 4-5 drain slots in them if you look at the pinch-weld flange on the bottom of the sill (outside the car). Make sure they're not blocked with any debris - if they are, poke a screwdriver or other flat object up in there and try to get the dirt out. Also consider removing the carpet and pulling out all those plastic sill hole plugs and vacuuming out any debris/leaves. The water from the plenum chamber below the windscreen flows out to the lower front door hinge pillar area then down to the sills. Leaves can also make there way in there and block the sill drain holes.
There are no drain holes in the floorpan under the carpet as Volvo never intended water to get in there. If the foam under the bitumen mats is wet, there's a water leak that needs to be addressed:
1) Windscreen
2) Blocked sills allowing water build-up and small leaks through the round plastic inner sill plugs
3) A/C evaporator drain hose from heater box through top of tunnel - blocked with leaves/debris and/or dislodged from heater box or tunnel and condensate water runs down under carpet when A/C is in use
4) Door wiring grommets dislodged
5) Wiper pivot mounts
That's a partial list!