So, after searching in cl_voice.lua for half an hour I noticed that in the vanilla terrortown gamemode this is already
implemented which I confirmed right away:



See cl_voice.lua script, line 68.



So I guess that your custom script is overwriting it or something.
Maybe you could just add those lines, test it out and see if it works like that or find what is overwriting it.

Regards
House

Edit: I'd personally prefer a darker blue tho.