Here are a few of the main options for allowing voice communication while in a Multi-User Session:
Radmin, if connecting over separate neworks, the preferred method is using Radmin (see this page). Here is a page also on communicating with audio while connected with Radmin
https://radmin-club.com/radmin-vpn/how-to-chat-in-radmin-vpn/
Discord: You can set up a Discord server and have users join a voice channel while they're in a session. See here for how to use Discord for Audio
Mumble: Mumble is an open-source, low-latency, high-quality voice chat software primarily intended for use while gaming. You can host your own server and users can join channels to chat.
TeamSpeak: Similar to Mumble, TeamSpeak is a voice chat service often used by gamers. It's not free, but it offers high-quality voice chat and the ability to host your own servers.