Unsure why but using the latest version of MC Eternal 1.4.4, this mod does not work, the menu option is there and it says it is outputting to my headphones but the sound continues to come out of my AV Receiver.
I have uploaded some screenshots on imgur.
https://imgur.com/a/riOadpP

EDIT: To clarify, i have not tried it on any other version of MC Eternal, that's just the version i am using.