It's great to see that GPLES is finally receiving some interest.
It took me several years to figure out how to add multiple engine sounds to the running GPL .exe so that the sim plays the sounds just like the normal, single engine sounds. When the patch was released back in 2014 and there was little to no interest in making new engine sound sets, I ended further development and and put it on the shelf.
I'm a bit rusty about how it works after all these years, but will try to help those who want to make new sounds sets and try the patch.
As Paul discovered, the dx8vb.dll is necessary for the sound editor and may have to be registered (it is not
needed if you just want to play the sounds in the sim). That is all covered in the Read Me.
The patch always uses the "Snd0" through "Snd6" sounds which must be located in the installation's sound directory. So if you want, you can use any sound files by manually renaming and copying them there.
However, the sound loader program uses the gples.ini file to determine when to play each sound so it's important that each mod and car have its own gples.ini file that is tailored for it.
If I remember correctly, the sound loader program does a simple check to see if you have a mod installed and its cars have a directory for its sounds. If not, then it won't show the mod's cars in the drop down list. I'll have to go back and see specifically what it checks for.