When you try to play note using Instrument.PIANO you will receive following exception:

??java.lang.IllegalArgumentException: Sound effect note.harp does not exist
at com.google.common.base.Preconditions.checkArgument(Preconditions.java:148)
at org.bukkit.craftbukkit.v1_9_R1.CraftSound.getSoundEffect(CraftSound.java:20)
at org.bukkit.craftbukkit.v1_9_R1.entity.CraftPlayer.playNote(CraftPlayer.java:296)
at com.verdent.wive.utils.effects.SoundUtil.playNote(SoundUtil.java:48)
at com.verdent.wive.engine.model.bdo.action.DelayStarter.countdownTick(DelayStarter.java:40)
at com.verdent.wive.engine.model.bdo.action.Countdown.process(Countdown.java:129)
at com.verdent.wive.engine.model.bdo.action.WiveAutomaticAction.run(WiveAutomaticAction.java:84)
at org.bukkit.craftbukkit.v1_9_R1.scheduler.CraftTask.run(CraftTask.java:71)
at org.bukkit.craftbukkit.v1_9_R1.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:350)
at net.minecraft.server.v1_9_R1.MinecraftServer.D(MinecraftServer.java:724)
at net.minecraft.server.v1_9_R1.DedicatedServer.D(DedicatedServer.java:400)
at net.minecraft.server.v1_9_R1.MinecraftServer.C(MinecraftServer.java:655)
at net.minecraft.server.v1_9_R1.MinecraftServer.run(MinecraftServer.java:554)
at java.lang.Thread.run(Thread.java:745)??

I haven't checked if it happens for other instruments too.