Asra29: I got it up and running just fine. Note that I am running the lastest version of Windows 10 x64. Here are the steps that I had to take in order to make it work properly:

1.) Install SoF
2.) Install SoF Plus to your Soldier of Fortune directory
3.) Make a copy of the SoF execuable (SoF.exe)
4.) Rename the copied SoF.exe to mohaa.exe
5.) Run SoF from the mohaa.exe for now on

I sent the mohaa.exe to my desktop and renamed it SoF and it works just fine for me.

EDIT - Forgot to mention something, this fix also works for Soldier of Fortune 2 Double Helix (Minus the SoF Plus mod installation / no need to do step 2 in order to fix Soldier of Fortune 2 Double Helix). Idk why it works that way, but it just does.

snorkel.216: I have the same issue.
I found a fix on the microsoft forums.

You have to make a copy of SoF.exe in the same folder
Run that exe and it works...

(On forum post it said to rename it to mohaa.exe but I've found that any name will work.
Don't rename the original as then it won't work.)

L33T_Taco: Evening Gentlemen, I made a account just to post my fix for the "defsnd.dll failed" problem that I "stumbled upon"

Step 1: leave the .exe files as "SoF.exe", don't rename it
Step 2: Copy "SoF.exe"
Step 3: Paste in the same folder which will create "SoF - Copy.exe"
Step 4: Run the copied .exe
I'm sure you can rename the copied .exe, not sure why this fixed it but it launched the game for me.