Automatically add the `sound` tag if the post has sound. Remove the tag if the post doesn't have sound.
A video is considered to have sound if its peak loudness is greater than -70 dB. The current quietest post
on Danbooru has a peak loudness of -62 dB (post #3470668), but it's possible to have audible sound at
-80 dB or possibly even lower. It's hard to draw a clear line between "silent" and "barely audible".
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