I noticed that if i set my speaker configuration as 5.1 or 7.1 some sounds will be missing or very lowered, especially the my character or vehicle's sounds. I also realized that this is a known issue in the GTAForum community.
I resolved this by using the quadrophonic setting, but, some people who costantly their speakers settings may find this a bit frustrating since they'll to change them everytime in order to play UG/UGMP.
https://i.ibb.co/Zch7SHP/a1.png