First, there is no Mi-28 in the SF2IO mod. You're referring to the Mi-24P.
Second, SF2NA is a minimum requirement for that Mod. If you don't have it installed, then you shouldn't be using the SF2IO mod. It's that simple.
There is a fix for the sounds issue with the Mi-24P and the AH-1F. See the attached, which should be unzipped into the \Objects\Aircraft folder in your SF2IO mods directory.
Really, though, you need to get SF2NA and get it installed. If you don't, you're going to encounter plenty of other problems with the SF2IO mod, including missing aircraft, and there isn't going to be a fix for those.
First, solve one problem at a time. That means get the missing aircraft problem solved first before you try to mess around with SweetFX.
Second, if you're not seeing any new aircraft, then you did not follow "all the installation instructions to the tee." The installation, in other words, is botched because the game isn't finding the new aircraft (and possibly some of the other new items from NF5) where it expects to find them.
I can't tell you where they are because I'm not sitting at your PC. I would recommend nosing around and finding where all the new aircraft are on your hard drive, them moving them to the proper location: \Objects\Aircraft. My guess is that they're buried in some sub-folder somewhere -- possibly \Objects\Objects\Aircraft or even \Objects\Aircraft\Aircraft.
The other possibility is that you dropped the new files in a mod folder that is not being used by the custom EXE that you created.
Wherever they are, they're not where they're supposed to be (or the game expects them to be). And the same likely holds true for the new Decals, Weapons, Guns, and Pilots as well.
