This is very similar to the process of making a TFA-VOX, so this should be a walk in the park.

You wanna grab a text editor like notepad++ and you need to download the example addon found here: https://drive.google.com/file/d/1BcmHgeE0GanG7Y-_OzQteEoehl924XRu/view?usp=sharing

Steps:
1. Use 7zip or winRAR to unpack the downloaded zip
2. Put your folder into Steam\SteamApps\common\GarrysMod\garrysmod\addons
3. Navigate inside until you find example.lua (helpful notes are left inside as guide)
4. Put in any sound path that you want to be subtitled
(you can find the path using a sound browser addon or by the ConVar snd_show 1)
5. Once you are done, rename your lua file to something unique.