I didn't investigate the licensing situation thoroughly but it looks like
this dependency is GPLv2. There may be a compelling reason this package was
excluded from Trisquel that I'm not aware of. If you're willing to dig in a
little bit to ensure the package isn't violating your freedom somehow, you
could always grab it from Ubuntu's mirror here:
http://mirrors.kernel.org/ubuntu/pool/universe/z/zeroc-ice/libiceutil35v5_3.5.1-6.4ubuntu1_amd64.deb
Then just install it with:
sudo dpkg -i libzeroc-ice35v5_3.5.1-6.4ubuntu1_amd64.deb
Then try to reinstall mumble server again:
sudo apt install mumble-server
Hope that helps.
