Velocity
You'll need extra ports for this tutorial. 1 for each server with voicechat plugin.
1. Every server on the same node
On The Backends:
Do everything stated here for each of your backend servers
On The Proxy:
Install SimpleVoiceChat for Velocity
The plugin will automatically detect the port of each voice chat server and will forward everything to the correct destination.
2. Servers on different nodes
Follow the steps for "Every server on the same node" and:
set voice_host on the backend servers to their (the backend's) actual IPv4