My ping goes up when I use the Garry's Mod voice chat

You will need to remove a few server variables from your server.cfg and then add a couple back.

1. Firstly you will need to remove the following server variables from your server.cfg file; sv_minrate, sv_maxrate, sv_minupdaterate, sv_maxupdaterate, sv_mincmdrate, sv_maxcmdrate

2. You will then need to add the following to the server.cfg:-
   sv_minrate 100000
   sv_maxrate 0

3. Save the server.cfg and restart the server, voila, its working again!

TIP: The server.cfg file is located under Configurations Files in the control panel