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Your vacuum readings are normal. Your ventilation system operating symptoms are indeed indicative of a vacuum leak somewhere.
If there is noticeable air divertor flap movement under the dash when the vacuum drops, such as during acceleration, then there is a failure somewhere in the climate control section at or after the check valve. The check valve can be found close to the intake manifold in a thin hose going through the firewall. Always start by checking the valve function. If it's just the check valve then that's not a huge issue, it just means it can't hold vacuum, they do sometimes fail.
However, in your case, because you hear hissing you obviously have a leak. You can verify the leak is in the climate control section by disconnecting the hose at the check valve and using mouth suction to see if the cabin system can hold vacuum, using your tongue to provide a seal. It should hold decent vacuum for say at least 20-30 seconds (preferably longer). Odds are of course for you it's going to fail that test, but at least now you're sure where the problem is. You will then need to open up the centre console to track it down and deal with it. Such vacuum leaks are mostly a nuisance, but it is still a source of unmetered air that the ECU has to try compensating which can affect idle, acceleration, mileage and emissions.
When there is a vacuum leak under the dash it is often most noticeable when the air RECirculation function is on and the fresh air shutter near the firewall slowly returns to the full open position under spring load as vacuum runs down, often creaking. That alone does not necessarily mean the leak is in the RECirc section, just that the system has lost vacuum. Now when the RECirc switch is off and the other flaps now operate normally when the engine stops providing vacuum under acceleration then the leak would be confined to the RECirc section. You can try narrowing down the hissing sound once you've got the centre console opened up.
I'll refer you now to Art Benstein's excellent and extensive 240 blower motor replacement writeup to help see what you're up against and what the climate control section looks like, http://www.cleanflametrap.com/ (Blower motor replacement - 1984 Volvo 244 DL). There are lots of pics which include the ventilation section. Most important for you is a link near the end to the climate and heater control section of the Volvo 240 Service Manual, 37 pages with complete diagrams. Although it's an early 1980s manual, there were precious few changes in later years. The control panel subsection starts at image 57. Unfortunately a few pages seem to be missing in that subsection, but there's still lots of valuable diagrams.
If you can isolate the hissing to one of the vent actuators then those can be replaced -you can temporarily plug off the hose to the offending actuator if you want. A quick search didn't show replacement actuators in the Volvo parts chain so you may need a donor car.
You may well find the hissing sound is coming from the back of the control panel. The leak may be at a hose connection, but more often is internally in a failing switch valve. You can try isolating the problem by selectively disconnecting hoses and plugging ports. I don't recall ever hearing that these switches can be overhauled. As I recall, the one leak I had there I just learned to live with as it only affected the RECirc function and the system would hold some vacuum except going up longer hills. The switches are not individually replaceable, you have to replace the whole unit (with a good used one).
I'll mention here that once you've got the console opened up, one of the easiest things to check for is a leak at the rubber hose connections on the ends of the vacuum reservoir, a long white plastic bottle that lives buried up in the centre console. The original ones were shaped like four tennis balls in a row, but I think that changed to a shorter, plain cylindrical bottle in later years. The vacuum reservoir bottle is not a common problem area in my experience, but a leak there can easily be fixed with something like RTV sealant.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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