You're not the first person that has made this question 'around here'. Check up this 'BBS post' and the response that it got. Perhaps, it could be the solution to your problem; https://www.newgrounds.com/bbs/topic/1489453
I hope that it results you useful. Have a good day/afternoon/night o7.
Yatsufusa
I am not deeply familiar with Scratch, but probably the same way you upload audio. If there are multiple files you should try uploading them in a zip-file and test how that plays out. (Click "Preview" instead of "Publish Project".)
eeboiee
Thanks!