First of all, everything works fine on my end. But the -i option just doesn't seem to do anything, I can put anything in there and the script will still work.
The option is only used (and required) if the ipv6 addresses of the speakers are link-local, i.e. start with fe80, correct? For me, this is not the case for some reason and it's weird that the option is still required even though it doesn't do anything.