Yeah it sounds like you are low on refrigerant. While the hissing sound could be from the actual leak, that sound would not be around long; a leak big enough to hear would lose the entire charge very quickly. More likely the hissing is the sound of what is left of the refrigerant flowing into the evaporator. The pressures are probably too low for the refrigerant to condense into liquid so you are just feeding gas through the system, hence, little or no refrigerating effect.
You will need to try to locate the leak. It could be a damaged evaporator plate, as DeniseO3O suggests, or it could be somewhere on the lineset between the compressor and the evaporator. Sometimes there will be an accumulation of oil at the leak, but not always. look for spots where the copper pipes could be chaffing on something. Sometimes on kit systems the excess line set is left coiled up in such a way that it can vibrate and chaffe together. you can use a slightly diluted dish soap solution on spots that you suspect might be leaking, and if it bubbles it is leaking.
If you can find the leak then we can talk about what needs to be done to repair it. There are a number of options ranging from a DIY repair attempt. (with our professional advice of course
) to replacement of the entire evap section. Or you might want to hire a professional in your area to repair it.
One note: do not run the system knowing it is low on refrigerant. Doing so will cause the compressor to run much hotter than it should. More importantly, the suction side could actually run in a vacuum, and if the leak is on the low side of the circuit it will draw moisture into the system, causing even more problems than you already have.