Go free to rate this as dumb if this is wrong but I might have an idea that might help you out. I have heard that fastDL sometimes does not fetch files and that it rarely works so I would try just for the sound parts to use:
resource.AddFile ( "dir here" )
I know this can be a slow download but this should mean that players will download the content as long as the addon is coded well and that you have the paths right. I may be wrong but I thought it can’t hurt and might help. If you have never used resource.add codes then what you do is you create a script under:
lua/autorun/server/(whatever you want to name the script).lua
The you would put it in a format sorta like this:
if (SERVER) then
resource.AddFile ( "file directory without the root locations like addons/blahblahblah" )
This was not very well said for how to use resource.addfile so if you need to know about how to I would just look up how to and keep this stuff in mind to fix mistakes.