Sound: there must be a way to hide a npub in here in a way that cannot be heard and cannot be filtered out. No? 
Discussion
Absolutly! audio stenography as really fun. it's not the most secure because anyone could run the file through a spectrograph and see what you imbedded in the audio. but they also have to know to look for it.
If they can find it, they can then filter it out… or replace it with another I assume no?
All this is way above my pay grade mostly trying to understand what is possible and what is not
Im not a pro either i've only messed with it a little bit. 😅 But from what I remember you can encode data into audio and then hide that audio in a "public" bit of audio that you could share anywhere. Like a song you really like or a ring tone. You can encode things above or below the human audible ranges or "broad spectrum" if you wanna be really sneaky lol. Then you or the intended recipient could decode the info at a later date.
My knowledge drops off a lot here but i'm guessing there are ways to detect irregularitys in audio files if you scrutinize it programmaticly. But I think someone would have to suspect that you are hiding something in audio to take a closer look at all your audio files.
https://github.com/swesterfeld/audiowmark
Problem you can't share the watermarking key or someone can remove the data. That's why they recommend embedding 'random' looking hashes and maintain a database that looks them up. But the readme on that repo is a good read in any case.