I jusr recently learned to do this and have been having a blast tweaking the sounds. I'ts actually pretty easy to switch the sounds and there are 2 ways to do it. Before you start you will need a few free programs. 1st is a prgram called "Eliteness", you can DL it in the Downloads section I think. 2nd is OPF Sound Lab, also easy to get. Now that you have these installed, you can get started. Open up Eliteness and browse to the PBO file for the addon you want to adjust. You should always backup the PBO in case you mess up. Once you find the PBO in Eliteness you can right click and select "unPBO", this will create a folder with the PBOs files available to browse. Now there are 2 ways to proceed. If the addon came with it's own custom sounds then you look for the files that end in .wss or .ogg inside that new folder you just created. These are the sound FX files for the addon. Once you find them use OPF Sound Lab to convert them to WAV files, its very easy. Once they are WAVs you can play them in any old media player to sort out the ones you want from the ones that need to go.
Now, to replace the sounds with new ones.....
If there are other sounds in the game, or from another addon you want to use you just find the sound PBO for those SFX and repeat the procedure above to get a hold of sounds you like better. One you have sorted that good from the bad you can rename the "good" ones to replace the "Bad" ones and place the good in the folder from the addon you want to modify. Remember you need to use OPF Sound Lab to convert them from Wav back to .wss or ogg. for Arma to be able to use them. So for instance, say folder "addon1" has a sound called "Gun1" that you don't like. Find a sound you do like, say "Shot1", and rename it "gun1" and replace the old "gun1" file with it so arma plays that new file. Once you have replaced all the sounds with the ones you want open up Eliteness and find the new Folder that you created out of the orig PBO, select the folder and chose "create PBO" , now it will make a new PBO file with the new sounds that you can place in the Arma Addon Folder as normal and fire up the game to test.
Now if the Addon you want to modify Does not contain it's own sound files then you can open up the PBO like before but find the config file and open that with eliteness, find the guns you want to modify and look for the sounds that are associated with them. You can copy and paste that names and pathways of new sounds. For instance, in the Config, the ak-47's sound is listed as ca/weapons/shot1.wss. or something like that, And you that sound you like is Shot2, just change that name of the file listed in the config and arma will use that one instead. Remember to create a new PBO when your done.
I hope this makes sense, get as many sound mods as you can and just start opening them up and poking around the config files to see how they are put together and you will see how easy it can be. Also, remember that Arma 2 uses that same wss. and ogg. file format as Arma1. So even though Arma 2 Mods don't work with Arma 1, the sounds themselves do once you unpbo them, it really is a goldmine of new sounds for your game. Also the free Program Audacity is a great way to modify Wav sounds, make them louder, deeper, or whatever you like.
Hope I explained this ok, have a blast playing with sounds, I know I am!!!!