If you're running macOS and have LabyMod voice chat installed, but no one can hear you, it's because Minecraft does not have permission to access your microphone. Unfortunately, Minecraft/LabyMod can't ask for the permission as other native apps can do.
Method 1 (recommended):
Install our "LabyMod Microphone Proxy" application. This native macOS application runs in the background and redirects your microphone input to the voice chat instance. You can download it here.
Method 2 (advanced):
We have put together instructions for manually adding the needed permission to Minecraft. Take a look here.
Make sure you first read the instructions carefully and follow it exactly. As it's required to go in the recovery mode to disable/enable SIP, it's recommended you open the instructions on a secondary device like your phone or another computer.