Does your AC clutch engage? Is the hissing noise present only when AC
compressor is running? If so than it is likely that the hissing noise
is due to the refrigerant moving through the evaporator. Some hissing
is normal, however, if the system is running low on refrigerant the
noise will be much more prominent.

You clearly have a leak in your AC system (may be your EVAP is  bad,
may be something else). I would strongly recommend that before ripping
the dash apart (I would really encourage you to get your hands on FSM
if you decide to do so (you can print out relevant sections of the
manual for $20 from www.techauthority.com)) you determine where the
leak in the AC system is. It is easier to do it if you have an
electronic leak detector, but if the leak is under the hood, then you
are likely to be able to find an oily spot somewhere on an AC
component pinpointing the leak. If you cannot find it, I would take it
to a shop so that they could pin point the leak. Regardless, before
attempting repair you would need to have a shop evacuate the remaining
refrigerant from the system. Then you can peform whatever AC repair is
necessary (don't forget to replace filter/driver) and take it back to
them to pull vacuum and recharge.

You can avoid the second trip to them if you manage to get your hands
on a vacuum pump, but if you need to buy one, it is likely to be
cheaper if you get a shop to do the refill for you -- do not just try
to add refrigerant to a system full of moisture -- very likely to
cause issues.

If you plan on getting rid of the car in a year or so and the leak is
in the LOW pressure side of the system (i.e. EVAP, return hose), you
might consider trying your luck with one of those STOP LEAK products.
I would NOT recommend this route as those products tend to be
problematic in terms of causing damage to other components of the
system and/or pose a serious maintenance issue.

If you decide to simply RECHARGE the system (with or without STOP
LEAK) be sure to get a product that includes proper grade PAG oil.
Remember, than when refrigerant leaks out, so does a part of the oil
that lubricates the compressor. Hence it is crucial to add oil along
with refrigerant.

Note: check your local area regulations -- in some jurisdictions
refilling the system without an attempt at fixing the system is
prohibited, some jurisdictions require a license for handling
refrigerants (even R134a) and/or prohibit sale of small quantities of
refrigerant (i.e. no cans).

Hope this helps,

Alex