Looping Sounds not working

I have a spell where after a two second delay a mushroom spawns

mushroom:EmitSound("Hero_Viper.NetherToxin.TI8")

Instead of looping however it ends after 8 in game seconds. I've tried other looping sounds and they fail to loop as well. Is there something special I must do to make a sound loop? I tried assigning it to a thinker but that didn't work out either.