Unable to load locomotives own soundset with DCS Loader

The original sound set should still be in the engine . did you  firing up the engine? Usually adding a new sound set just writes over top of the old one. If it fails, the old  sound file  sould still be there depending at what  step the loader failed.    I'm still using an older version of the loader. version  5 i think. I'm not familiar with chain files etc. I would trt starting up the engine and see what happens.

The original sounds still work. It just won't let me add a new set. I wanted to see if it would accept it's own set of sounds from the MTH website and it still wouldn't accept that file either. It's not a huge deal, just wondering why it won't accept any of the files.

You have an older 3V board with 1MB of memory.  I suspect that fixes were made to the original sound file that pushed it over 1MB.  Contact MTH support and they should be able to get you a sound file that will work.