Low refrigerant was the culprit. 
I added about 3/4 of a 12oz can of R134 and there is no more hissing, not even when the compressor kicks in, and my A/C is once again ridiculously cold.
For anyone who wants an easy, 10-minute DIY solution, below is what I did.
Note: It took longer to write this post than to do the recharge.
1. Go to Wal-Mart and get the
Interdynamcs EZ Chill Trigger Dispenser ($15.99).
*** DO NOT buy ANY variety of Interdynamics R134 since they all contain sealants that could kill your A/C system, void its warranty, and which A/C professionals will not touch if you need their help. Instead, get a can of virgin R134a – my Wal-Mart stocks just one, made by Johnsen’s ($6.77).
2. Find the low pressure A/C fitting on the pipe leading to the Receiver bottle. On my car it is in plain view on the driver’s side, with a blue cap. Unscrew the blue cap.
3. Start the car and use a tool bag or other heavy object to depress the accelerator enough to maintain 1500rpm.
4. Connect the Trigger Dispenser hose to the low-pressure fitting. Once secured, you will get an immediate pressure reading. If you do not, your connection may not be secure. If the connection is secure, and you get no pressure reading, you should probably stop what you are doing and seek professional help since your a/c system is depressurized.
(My reading was 80psi with A/C Off.)
5. Turn the A/C on max, blower on full, recirc on. Check reading again. It should have dropped significantly. (My reading dropped to 10psi with A/C on.)
6. Detach the Trigger Dispenser and tightly screw on the R134a can. This will pierce the can.
7. Reattach the Trigger Dispenser hose to the low pressure port. Make sure it is secure. Squeeze the Dispenser trigger to begin the R134a flow.
NOTE: When you squeeze the trigger, pressure on the gauge will drop to zero. Let go of the trigger and you will get a pressure reading of the A/C low-pressure line. While recharging, you can periodically stop and check temperature from the dashboard vents. It should be noticeably cooler almost immediately.
Continue adding refrigerant until you approach 25-30 psi. That seems to be the target range, based on what I have read in various threads here. That opinion is corroborated, at least in my case, by the soft “thump” (for lack of better terms) I heard from the compressor when pressure approached 30psi, and its spinning noise became more muffled, as if it was now under a heavier load. I stopped at about 25-28psi (assuming the dispenser gauge is accurate).
8. Detach the Dispenser hose and screw the blue cap back on. Go for a drive. If you were experiencing expansion valve hissing it should gradually disappear as more of the refrigerant liquefies. After a 15 minute or so drive, there will be little or no discernible hissing.
After the drive, check pressure once again. (Mine was now at 32psi.)
If you are sure you only need a recharge, and have no major leaks, a DIY r134 refill is an easy solution.