Make sure you have this know you said it was working yadda yadda but double checkerino.
Note: CEF is for most of the functionality on the server, Media Stream is for voice chat usage.
How to enable on client:
Set DisableCEF to false and MediaStream to true
Example code for settings.xml (Found in GT-MP folder)
<DisableCEF>false</DisableCEF>
<MediaStream>true</MediaStream>
Copy pasted instructions check.
Beyond that when you load up hit p twice before selecting a server (Where Micheal is talking to his shrink in the background) this will open the gtav options menu.
Check in your game options that your microphone is set to enabled and all that good shit.
Can also do a good ol /vv100 set voice volume to 100 incase its super quiet and you are just not hearing it or something.
Could also be some serious desync , hasn't been near as bad as it once was but failing everything else being a-okay could be that mucking up.
If none of that works let me know I will have a beer and a think on the matter see if I can think of anything else.