Mumble is a free, open source, high quality, and low latency voice chat software. Primarily intended for gaming, it is also commonly used for recording podcasts and collaborating on software development (among others).
Downloads for various operating systems can be found on the Mumble website. For most Linux distributions, you can also just use your package manager: for Arch,
sudo pacman -S mumble, for Ubuntu,
sudo apt install mumble, and so on. I've had numerous issues with Plumble on Android but I also know other people that are using it without any hitches ¯\_(ツ)_/¯ The best experience is on desktop.
Launch the application then go through the Audio Wizard. It will help you identify most issues and get them resolved before connecting to the server. Once that's finished, a dialogue should pop up prompting you to do just that. Click Add New..., enter
nixnet.xyz as the address, leave the port as default, enter the username you'd like to use, then give the server a friendly name with the Label box. You'll be able to quickly connect to the server again by clicking it as an option under