The files for the hitsouds start with normal, soft, and drum. There are 7 sounds each (hitnormal, hitfinish, hitclap, hitwhistle, sliderslide, slidertick, sliderwhistle). Filenames are combinations of these, such as normal-hitnormal.wav or soft-slidertick.wav, and you should have all 21.
I personally use the hitsounds from -YUGEN- because that was my main skin for a long time, but I changed them so that all of the hitsounds are the same as normal-hitnormal except for the slider's sounds. For those I left the slidertick and replaced the other 2 with blank .wav files. You can just use the default -YUGEN- sounds though, those are good.