My bungalow has the standard wall/cavity ventilation blocks
strategically placed in the walls. Trouble is that one of these is
about 5 feet off the ground and doesn't even have any hooded cover on
it, so whenever the wind blows very strongly from the right direction
and it's raining, rain is blown straight into the cavity. This is of
course a bad thing, but thankfully the wind doesn't often blow
strongly enough from the relevant direction.

Should I just get a 'hooded'/slatted cover and install it over the
existing slotted ventilation 'brick' ?

Also, opposite this on the inside wall (the room is the lounge) there
is a slatted plastic cover - the previous owner had put a layer of
corrugated cardboard (!!!!!) inside this cover in an attempt to
prevent outside sound from getting into the room. A piece of cardboard
isn't particurlarly effective though as I'm sure you'll agree! So is
there anything better that I can use for good sound-proofing, or
should I just properly block off the 'inner' ventilation slot? After
all, surely it's not needed just so long as the ventilation gets to
the cavity via the *outside* vent?

Thanks