normal-hitnormal.wav
normal-hitwhistle.wav
normal-hitfinish.wav
normal-hitclap.wav
normal-sliderslide.wav
normal-sliderwhistle.wav
normal-slidertick.wav
Put files with these names inside your song folder, then under Timing Setup, Audio tab, select Custom 1. For custom Soft Sampleset sounds, do the same, but use soft in place of normal in your filenames. To make use of that Custom 2 option, add the number 2 at the end of each filename (just before the .wav)