The Problem
Adding custom sounds to FiveM in a way that works with natives like PlaySoundFromEntity is typically a lot of work.
You need the audio in a very specific file format, create configs, convert them using CodeWalker and OpenIV, …
The Solution
But there is an easy converter tool, where you can just select any audio file you want, give it a name and it spits out a fivem resource that adds it to your server.
You can find it here:
And then you can easily use it in your scripts using any of the games sound related natives.
It also gives you a readme file with all the info you need for that.