Here is a log of the error I'm seeing on my server quite often:

[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: java.lang.NoSuchMethodException: net.minecraft.entity.EntityLivingBase.func_184601_bQ()
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at java.lang.Class.getDeclaredMethod(Unknown Source)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at mchorse.metamorph.util.InvokeUtil.getPrivateMethod(InvokeUtil.java:53)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at mchorse.metamorph.api.morphs.EntityMorph.getHurtSound(EntityMorph.java:785)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at mchorse.metamorph.entity.SoundHandler.onPlaySound(SoundHandler.java:76)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_1323_SoundHandler_onPlaySound_PlaySoundAtEntityEvent.invoke(.dynamic)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:182)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraftforge.event.ForgeEventFactory.onPlaySoundAtEntity(ForgeEventFactory.java:498)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.world.World.func_184148_a(World.java:1121)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.entity.player.EntityPlayer.func_184185_a(EntityPlayer.java:403)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.entity.EntityLivingBase.func_184581_c(EntityLivingBase.java:1118)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.entity.EntityLivingBase.func_70097_a(EntityLivingBase.java:1028)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.entity.player.EntityPlayer.func_70097_a(EntityPlayer.java:1038)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.entity.player.EntityPlayerMP.func_70097_a(EntityPlayerMP.java:645)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at micdoodle8.mods.galacticraft.core.entities.player.GCEntityPlayerMP.func_70097_a(GCEntityPlayerMP.java:68)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.entity.EntityLivingBase.func_180430_e(EntityLivingBase.java:1325)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.entity.player.EntityPlayer.func_180430_e(EntityPlayer.java:1946)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.block.Block.func_180658_a(Block.java:821)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.entity.Entity.func_184231_a(Entity.java:1132)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.entity.EntityLivingBase.func_184231_a(EntityLivingBase.java:215)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.entity.player.EntityPlayerMP.func_71122_b(EntityPlayerMP.java:838)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.network.NetHandlerPlayServer.func_147347_a(NetHandlerPlayServer.java:585)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.network.play.client.CPacketPlayer.func_148833_a(SourceFile:126)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.network.play.client.CPacketPlayer$PositionRotation.func_148833_a(SourceFile:18)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.network.PacketThreadUtil$1.run(SourceFile:13)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at java.util.concurrent.FutureTask.run(Unknown Source)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.util.Util.func_181617_a(SourceFile:529)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:723)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:397)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:668)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526)
[23:28:35] [Server thread/INFO] [STDERR]: [mchorse.metamorph.api.morphs.EntityMorph:getHurtSound:795]: at java.lang.Thread.run(Unknown Source)

This Metamorph build should get rid of the error:

https://github.com/asanetargoss/metamorph/releases/tag/hurt-sounds_1.12

I guess it got resolved?