Solved Playing sounds to everyone on the server?

Custom no, existing effect yes. See Using game particles / effects (C#) | Oxide.

 

Also, found this that may help:

Code:

EffectNetwork.Send("assets/bundled/prefabs/fx/gas_explosion_small.prefab");

I haven't tested that, but it's a slightly different call than the others linked. There does appear to be a bool that might be set in prefabs or somewhere for "broadcast", which I'm not sure how to change.