Just for the record if anyone else gets stuck.
I couldn't get sdvx 6 to run because of that error where it fails to boot because it can't "find" soundvoltex.dll. Installed all the redistributables, and also added the x64 directx d3d9.dll and it STILL didn't work. I'm on AMD 5700xt.
Looks like the soundvoltex dll has some dependency on nvidia cuda dlls. I just grabbed nvcuda.dll and nvcuvid.dll from one of those slightly sketchy looking sites and tossed them into the content folder. Obviously it doesn't let you use CUDA on AMD, but the game reads and moves on from that error.
Hope this might help someone else that had that same problem.