From what I learned, the system is used to detect audio submissions using unoriginal work. This is to prevent users from submitting content that did not make, including royalty free submissions, since it is against the rules. There are a lot of users who submit either remixes or straight up stolen content from royalty free sources so this was developed to combat against that.
However, I think using royalty-free soundtacks would be fine to use as BGM for voice submissions and podcasts. I believe that this system is automated, so it may be a bug. Keep in mind that this system is still pretty new and the Newgrounds team is quite small, so there will be a bit of work needed before these issues are ironed out.
I think you can contact an audio mod to sort things out, here's a list of mods:
https://www.newgrounds.com/wiki/help-information/site-moderation
AmmoGuppy
Woah. I have been noticing other people having issues on the audio portal in the same boat as you recently. I cant say if its over zealous people or a messed up bot. Can you appeal? Maybe you can get a hold of a moderator in the audio forum to see whats the real reason. Hopefully its just a misunderstanding and things can be cleared up soon. Best wishes!