Are you sure you installed everything correctly ?
Are you sure the mod is not in different parts and need to download another part of this mod which contains the sounds ?
Large mods download are mostly not in 1 single part as it should mean a very big download ... taking too much time for some with a slow internet connection.
Large mods may mean: .... more than 1 or 2 GO.
I don't know which mod you are talking about so I can only give you a generic answer :
- some modders split their mod in several parts sometimes due to large archive so you may need to download CARS + SOUNDS + TRACKS as individual archives then extract all 3 parts in your game folder to restore full mod
- sometimes modders use 3rd party mods which are not included for any reason (reduce mod size or lack of permission) so you need to find them and install them by your own
So my advice is to read either readme or install instructions where you have downloaded your mod...if there are none then it is a really bad mod made with feet LOL