Guns that weren't loud enough
The AK-47, VSS Vintorez , AS "VAL", AK74, AKs74u, FN FNC, H&K G3SG, AMT Automag, Winchester model 12, Lewis Machinegun and Desert Eagle all fall under this category.

• AK-47 : it's a damn AK-47, not a silenced repeater. How can the silenced AK-47 be louder than its normal counterpart?
• VSS Vintorez and AS "VAL" :  because they share the same files, and even though they're silenced guns by default, i gave them more presence and a bit more loudness.
• AK74 and AKs74u : same reason as above.
• FN FNC and H&K G3SG : just not enough presence for automatic weapons.
• AMT automag : sound didn't seem right for a revolver of this caliber.
• Winchester model 12 : I expect my shotguns to sound badass and loud.
• Lewis Machinegun : louder, more bass, weird delay fixed, what could you want more?
• Desert Eagle: gave the DEagle the more "oomph" a gun of that caliber deserves.
• LaFrance M16K : it was already loud enough, just fixed some weird echo it had.
  

Guns that needed a new sound
The Colt-pocket 1849 , Duplet shotgun , FN1905 pistol, Beretta target 87 and Colt AK112 fall under this category.

The C1849 because its sound effect was more like someone banging on a carboard box with a stick than a real gun shot. I changed its sound for the .357 magnum revolver one.

The Duplet shotgun needed a new one because, with all due respect to millenia, it was not good quality and not loud. I couldn't work with the file too because of the poor quality. I combined multiple sound files to create my own custom sound, but I also provide the Ithaca sound effect in the optional files if you prefer that one.

The FN1905 and Beretta target 87 pistols had a weird sound that just didn't seem right. I gave the Beretta the sounds from Millenia's Colt1911 and the FN1905, the Mauser C96' sounds.

With all respects to Tactaghoul, the Colt AK112 original sound was horrid. I substituted with one from Millenia's weapons.




Automatic Guns with clipping sound
After all the research i've done, I am pretty confident to say I am the first one to ever document this bug - or I'm just so picky about sounds that no one else notices it but me.

The most noticeable example of this bug is the infamous PPSH-41. You can hear it for many, many more automatic, high fire-rate weapons:

• AK47
• AK74 & AKS74U (sharing the same sound files) and also made these ones louder
• Homemade SMG Borz & Sten MK II (sharing sound files with IMI UZI)
• PKM machinegun
• H&K G36K
• Vanilla's 9mm SMG
• Suomi kp-31 and its variant
• CZ 805 BREN A1 / A2
• Ruger SR-556
  

From the list before, I decided to only fix the ones I put in green. I judged that the other ones were too subtle to really be annoying on the long term.
(The 9mm SMG fix is in the optional files as a standalone)

I am still not certain what the origin of this issue is - I highly doubt it was an intended "cool effect" introduced by Millenia.
My intuition was that because it would only do it with high fire-rate weapons, the amount of data from the sound files was too much and the engine couldn't handle it all at the same time - hence cutting the sound effect before it has a chance to be played. After playing around with the files and noticing that some were really unoptimized (e.g. half of the file is complete silence at the end), I cut some of that unwanted file space and sure enough it did the trick for each of them all!
I have no proof for this, only my own experimentations.
If you have further knowledge on this, please bring your help :)

All of this was done by using Millenia's original sound files (i have used mine in case i went for another sound).
I used:

• Audacity software to edit the files
• A quality pair of headphones (not speakers!)
• The following in-game sound settings: