A great alternative to vivox is mumble. It's changed a lot since the mumble-whisper project for opensim back in 2010. I run a mumble server right here at my house, and anyone can connect to it and try it out. On windows just download the mumble client https://dl.mumble.info/latest/snapshot/client-windows-x64 or on apple mac download this client https://dl.mumble.info/latest/snapshot/client-macos-x64
Mumble also has clients in most linux package managers already ready to go. Mumble has super high voice quality and it's totally private. I've already used it at my grid with several people and the quality was amazing. Plus the fact the mumble server runs on a small raspberry pi at my home with zero problems. If you wish to connect to it, just use cyberdatastorm.com as the server address and you can try it out on port 64738. There are also mumble 'servers' for windows, mac os, and linux. So you can run it on something besides a raspberry pi, and have your completely secure and private own voice server. I understand a lot of people worry about vivox listening in to their private conversations. ok. Here's an alternative. What's great about it is you can play opensim with mumble in the background so it doesn't affect the game play at all, AND it's great voice quality.