Overall Garry's mod lua/practical tips and tutorials
Using souns for MQS and Simple NPCs
Using custom sound files
First of all your custom sound file must be uploaded to your server content (like workshop dl or fast dl) so other players can download it to hear it in the game. The file type can be rather .mp3 or .wav. For the test, you can add this sound file inside your local game folder. Like this - SteamApps\common\GarrysMod\garrysmod\sound
After you test your sound, put it in the sound folder of your server content folder and upload it.
Here is one way to do that, thanks to DanFMN
Note that you can sort your sound by the folders. Just create a folder inside your addons sound folder and put the sound inside, and avoid using spaces in the folder name.
If I put the file inside GarrysMod\garrysmod\sound\ and the file name is test.mp3 in the path for the sound you type only test.mp3
If I put the file inside GarrysMod\garrysmod\sound\mysounds\ and the file name is test2.mp3 in the path for the sound you type mysounds/test2.mp3
Using sound URL
To use a sound URL instead of the sound path, simply copy-paste a sound URL instead of the sound path.
The sound URL must be direct, you can't use youtube or any other streaming services to play sounds. But you can play radio steams.
Here is an example of a direct sound URL:
You can use Discord to host your sounds and play them in-game. To do that send a sound file to any DM or server text channel. Right-click the file name and select the Copy Link option.