Good idea–I tested it, with a surprising result:
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In the blocked onebox domains settings, I entered: www.dropbox.com, https://dropbox.com, dropbox, https://www.dropbox.com (I wasn’t sure what the right variation was, so wanted to cover my bases).
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After I did that, audio files did not onebox or convert into a (unusable) audio player–they just stayed as the dropbox link. So long as dropbox won’t play audio on non-dropbox sites, this seems like the best result for audio.
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But: Other dropbox oneboxing still seemed to happen (surprisingly), even with dropbox entered in the blocked onebox domain settings. For example, pasting in a dropbox image link still showed the image itself in the discourse post.