chris4, you're a senior member and have been here for quite some time. Thus, you should know that you first must search before suggestion something. As it happens, this has been suggested several times before (and as a matter of fact repeated again only recently)...
Anyways, Patchou is currently looking into how to add custom categories. Though, this isn't quite so easy as it sounds and it involves many things to take into consideration. It is on the to-do list so it might be added in the futur...
For the time being I suggest to rename your custom sounds so they are sorted properly (by name).
eg:
Simpsons: blah blah
Simpsons: yadda yadda
Simpsons: blubber
SouthPark: more blah blah
SouthPark: and yadda yadda
etc...
/ 
