'Did something like this for my game, except for backgrounds.. let me go check how it worked.
Ah, here it is:
strFolder = "Sound"
strFile = "mysound.wav" // just an example
sound_add(working_directory + "\" + strFolder + "\" + strFile, kind, preload)
If you don't want to use those extra variables, it'll look like:
sound_add(working_directory + "\Sound\mysound.wav", kind, preload)