when you use /splay, If you don't specify a folder, the sound folders in the Sound Requests dialog are searched. do you have the correct directory set there?
I know the correct usage of /splay, it just returns unable to play. If it helps any, this occurred after I used NoName Script, and it's mp3 player uses Amp_in.dll
since /splay worked fine before you started using that script, and wont work with that script, have you asked the ppl who wrote that script/dll about any known issues or incompatibility?