These sounds are meant for web apps, right? I would imagine they will wind up in a source control repository for a web app.
So there's only one individual that's allowed to add them to the source control repo?
The license says "It is unlawful to distribute any of the audio files to ANY ADDITIONAL USERS", does that mean it's unlawful if other individuals have access to that repo and check it out? Or they can check out the repo, but they can't... write code that uses the sounds added to the repo, only that one guy can write code? Can another individual fix bugs in his code? Can another individual deploy the code? Or to the other extreme, as long as you have one individual committing all the sound files to the source repo, the entire rest of the company can then write code using them?
None of those make much sense, I can't think of anything else that does.
But if Boom Library has the same sort of license and has been succesful, then either it makes sense in a way I don't understand... or nobody uses Boom Library for projects from companies or team entities, just one-man operations... or everyone's just ignoring the license and doing what they think probably makes sense.
Looking up the Boom license, it does say "If you want to purchase a multiuser license, please contact us directly." So I'd guess some are doing that, and others are probably ignoring and violating the license.