More-Planets-1.12.2-2.0.21
Sounds of mobs added by the mod fail playing while being on a Multiplayer Server.
The following Report was generated after coming across a Cheese Floated but the same thing also happened with Alien Miners.

Notification by VanillaFix : https://paste.dimdev.org/xadagarema.mccrash

Seems work fine to me, Cannot reproduce. (Multiplayer test)

I have had the Same issue, will investigate further later on.
But I think, it's because you check in your registerSounds() method, whether you are on a client, which doesn't make sense, because you have to register the sounds on the server as well.
If you don't register them on the Server, it can't resolve the Sound ID and then wrong packets get sent to the Client (specifically the Packet, which plays Vanilla Sounds). But because your Sounds aren't Vanilla Sounds - obviously - VanillaFix Shows the notification

Oops, it seems missing RegistryEvent for SoundEvent.

Fixed on both versions. 2.1.9 and 2.0.22.