A wooden harp is a tonal musical instrument, and one of the two harps. It can be used to play the passtune to enter the castle. Improvising with it will exercise your dexterity, and possibly make nearby nymphs peaceful. Nymphs will be made peaceful if you are not confused and your experience level plus a random number between 0 and your dexterity is at least 26. Therefore pacifying nymphs can only work reliably if you are at least level 26.
It will either play "a lilting melody" (which pacifies nymphs) or else it "twangs." If a harp produces "very attractive music" it is a magic harp instead. If you choose not to improvise, and the harp produces a "strange sound", then it is a wooden harp.
A samurai knows the wooden harp as a koto.