If you want to add a custom M60 sound, then use "EarShot" mod. Installation is simple. I extracted the M60 sound from GTA VC for the best match with the in-game atmosphere.
EarShot: https://www.mixmods.com.br/2022/05/earshot/

There's a readme file inside that mod, read that thoroughly, and you will understand how to install that. Come on, you are a genius!

MeowX
05th October, 2023