ENB is a series of graphics mod to make the game look better. the first advice I'd give you is backing up your entire game folder before starting with modding. Mods usually overwright original files and if you don't backup .. u'd have to reinstall the game when you're not satisfied with the mod.
Now for sound mods, you first need to install something called GtaIVAUD
If you haven't already, you should also get either Spark IV or OpenIV (or both) these are programs for opening up your gta game and exporting (or importing) files
I suggest getting SparkIV, it's somewhat more user friendly (in my opinion)
(not sure if it's the correct version)
Now what you want to do is open up your game with SparkIV (or openIV) ,hit browse. Then navigate to pc/audio/sfx .. in there you'll see (on the right side) a file called resident.rpf, open it. (double click)
Then click resident in that file
You'll see a list of files (weapons, explosions etc) now let's say you want to install gun sounds. Export weapons.ivaud to your desktop (or any other location) open it up with gta-ivaud. Now you can import sounds 1 by 1 or .. you can replace the entire weapons.ivaud if a mod provides it (if so .. you can skip the exporting and just import the new weapons.ivaud).
If you're done editing the file just import it via sparkIV and hit save. Now you're done
If you want a suggestion for a weapons soundmod, I'm very satisfied about the one I have which is this one:
I personally don't like the melee sounds, but the explosion and gun sounds are very cool. For this mod you don't need gta-ivaud, you can simply open up resident.rpf like I said, click resident and import the files and then save.