At a guess, it might be that the audio URLs were on www.dropbox.com and being blocked by the first thing you entered while the images were on dropbox.com or somethingelse.dropbox.com and not being blocked by the other values you’ve entered.
It’s not completely clear from the description but that setting applies to the domain entered and all subdomains. So entering a value of exactly dropbox.com will block dropbox.com, www.dropbox.com and anythingelse.dropbox.com