SFX for items successfully/failed smelting.

The sound could apply to all types of furnaces. First of all, the successful smelts.
Keep in mind this is when there is nothing left to be smelt.
Smokers could have a hissing sound, similar to a grill.
Blast Furnaces could have a fwoosh. 
Furnaces should crackle after finishing.
If the furnace(s) ran out of fuel, it sounds like a dying fire, or even the sounds above if you can't think of anything else.