Do you mean the sounds or the actual emoticon? 
If you want to change the emoticon just type /skiss (or any sound) 

 or wahtever.
If you wanna make new sounds you can go 
Here and download the audiator 3 plug in and go 
Here For help about it. (FAQ) 
***your contact must have this plug in install to hear the sounds aswell***
Edit: and you could probably make quick text with it...like, it if were a sneeze, make a custome emoticon sneezing (kinda) and then add the sound to it...so like "/sneeze 

 *achoooo*"