Hissing sound is typical of refrigerant leak. You should first check the refrigerant level, if the pressure is significantly low: there could be a leak.
If refrigerant level is good, then the fault could be with the compressor. But, before checking the compressor, you should first check if the mechanic re-installed the serpentine belt properly. Improper serpentine belt fitting can also cause AC to not function properly, as the belt also rotates the compressor. Typically when there is issue with serpentine belt, there are other symptoms as well, like power steering not working properly or engine overheating.
