The amount of wind noise in my 242 has bothered me since I bought the car, and though I can not say that I have solved the problem, I have made a major difference with just a few simple upgrades. Though I have never had the chance to drive a post 91 240, from junkyard analysis I would say that Volvo has at least made a good attempt to solve the problem, and due to the similarity of 140 and 240 series cars most of the improvements can be adapted to an earlier 240 and sometimes the 140's.

Windshield
The post 90 style windshield with the flush trim makes a significant reduction in wind noise, especially at freeway speeds.

Shifter insulation/Shift boot
A surprising amount of heat and wind noise can enter the cabin through this relatively small hole. I used to be able to warm my fingers in winter by holding them between the radio and shifter. The stock rubber accordion type shift boot helps, so make sure yours is in good condition. Adding the leather shift boot on top will help a little more (only $70 at your dealer!). The real solution on later 240's is this piece of foam insulation (pictured below), which can easily be added to earlier cars and probably even 140's or P1800's with the short shifter.

PN 1371737 is the version for the M46 and M47 transmissions, and there is another for the automatic transmissions. Your dealer should be able to find the numbers for you, if they do not have them in stock. Installation is simple: from the cabin, remove the shifter boot(s) and remove the overdrive wires if you have an M46. Then simply stuff the foam down the hole, bend the metal clips over to keep it from falling out, and replace the overdrive wiring and shift boot(s). I have seen several in junkyards and they do hold up remarkably well.

Door weatherstripping
This is a half hour job at the most if you have a 244 or 245, and only slightly harder if you have a 242. Post 87 240's have an additional length of weatherstripping around the tops of the doors, attached with double sided foam tape. Gently peel them off the donor (careful where they curve), then clean and attach them to your doors. 3M makes an automotive grade 1/2" foam tape that works quite well.
If you have a 242, grab the seals for the front two doors and one of the rear doors from the donor. Start at the rear of your door and tape an entire front door seal down securely. Then measure the remaining distance and cut a piece to fit from the rear door seal you picked up. Tape it in place, and do the same for the other door. You will be amazed at how effective this is, especially if your door frame has been bent back by someone trying to get into your car without keys.

I have also added this seal to the doors of my 80 Bertone using the same procedure.

Bumpers
The smaller, lighter bumpers (EU81-, USA83-, CAN 85-) not only look better and weigh less, but they also generate less drag and less wind noise. I have heard people say that the earlier bumpers are better in accidents, but I have not yet seen any actual evidence to support this, nor does Volvo say so. In all fairness they may be, so consider yourself warned.

Door foam
A foam strip at the top of the door panels cuts down on wind noise and vibrations. USA-80 240's do not seem to have them, but they are simple to add. 242 versions in decent shape are rare at best. I have used a pair from a 244.

Side mirrors
The more streamlined side mirrors from 86-93 240's create less wind-noise than the early bmw style power mirrors do. A couple holes needed to be drilled or enlarged to add them to my 81, but the job isn't too hard.

Snow cap
I love mine, but it makes a heck of a lot of noise at 70mph. Makes just as much noise with a 240 hood as with the 260 hood.

Hood insulation
Engine noise can be even more annoying than road noise, especially if you still have a fan clutch. 240 Turbos and Diesels came with factory hood blanket insulation that fits the 81-85 USA hood, "260 hood" in the rest of the world. Though they are rare in the US, there is a version that fits the USA86- hood. 240 Classics, some 92 240's and most Polar and Super Polar have them. I think 240SE's do too.
IPD also sells a version that fits some 240's.

Rear wheel arch foam
76-81 262C's, 264 GLE's and 89 or so and later 240's have two triangular chunks of foam shoved in the gap above the rear wheel wells to insulate the cabin from trunk noises (and smells). I have not noticed a significant reduction in noise with these, but they weigh nothing and are simple to install.

Bertone trunk carpeting
The trunk is one very large echo chamber, and the Bertone carpet panels not only look very good, they also quiet things down back there. Two different versions are available, -78 and 79-. Cosmetically one of the nicest upgrades you can do to a 242/244's trunk. 86+ trunks are the same as the 79-85 trunks, except the outer skin, so the 79-81 Bertone carpet actually fits 240 trunks from 79-93.

Under dash insulation
240 Diesels have an extra inch of insulation glued to the back of the standard under-dash mat. Unfortunately 240 Diesels were rare when new, and super rare 20 years later, so they are hard to find, but it would be easy to make your own.

I am interested to find out if the new dash with knee bolsters is better insulated than the old dash and flimsy under-dash mats. Fitting the newer dash into an older car is an involved job, but not terribly difficult. I hope to do that in the next year or two.

Firewall insulation
If yours is missing, you'll want it.

Electric primary fan

I found that eliminating the whoosing of the clutch fan when revving the car made it seem much quieter.