posts: automatically tag videos with sound.

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".

app/logical/exif_tool.rb

@@ -124,6 +124,11 @@ class ExifTool
         metadata.has_key?("PNG:Dream")
     end
 
+    # True if the video has audible sound. False if the video doesn't have an audio track, or the audio track is inaudible.
+    def has_sound?
+      metadata["FFmpeg:AudioPeakLoudness"].to_f >= 0.0003 # -70 dB
+    end
+
     def width
       metadata.find { |name, value| name.match?(/\A(File|PNG|GIF|RIFF|Flash|Track\d+):ImageWidth\z/) }&.second
     end

app/models/media_asset.rb

@@ -22,7 +22,7 @@ class MediaAsset < ApplicationRecord
   has_many :ai_tags
 
   delegate :frame_delays, :metadata, to: :media_metadata
-  delegate :is_non_repeating_animation?, :is_greyscale?, :is_rotated?, :is_ai_generated?, to: :metadata
+  delegate :is_non_repeating_animation?, :is_greyscale?, :is_rotated?, :is_ai_generated?, :has_sound?, to: :metadata
 
   scope :public_only, -> { where(is_public: true) }
   scope :private_only, -> { where(is_public: false) }

app/models/post.rb

@@ -402,7 +402,7 @@ class Post < ApplicationRecord
     end
 
     def add_automatic_tags(tags)
-      tags -= %w[incredibly_absurdres absurdres highres lowres flash video ugoira animated_gif animated_png exif_rotation non-repeating_animation non-web_source wide_image tall_image]
+      tags -= %w[incredibly_absurdres absurdres highres lowres flash video ugoira animated_gif animated_png exif_rotation non-repeating_animation non-web_source wide_image tall_image sound]
 
       if tags.size >= 30
         tags -= ["tagme"]
@@ -472,6 +472,7 @@ class Post < ApplicationRecord
       tags << "exif_rotation" if media_asset.is_rotated?
       tags << "non-repeating_animation" if media_asset.is_non_repeating_animation?
       tags << "ai-generated" if media_asset.is_ai_generated?
+      tags << "sound" if media_asset.has_sound?
 
       tags
     end

test/unit/post_test.rb

@@ -1264,6 +1264,24 @@ class PostTest < ActiveSupport::TestCase
         end
       end
 
+      context "a silent video with the sound tag" do
+        should "automatically remove the sound tag" do
+          @media_asset = MediaAsset.upload!("test/files/mp4/test-silent-audio.mp4")
+          @post.update!(md5: @media_asset.md5)
+          @post.reload.update!(tag_string: "sound")
+          assert_equal("animated tagme", @post.tag_string)
+        end
+      end
+
+      context "an audible video without the sound tag" do
+        should "automatically add the sound tag" do
+          @media_asset = MediaAsset.upload!("test/files/mp4/test-audio.mp4")
+          @post.update!(md5: @media_asset.md5)
+          @post.reload.update!(tag_string: "tagme")
+          assert_equal("animated sound tagme", @post.tag_string)
+        end
+      end
+
       context "a post with a non-web source" do
         should "automatically add the non-web_source tag" do
           @post.update!(source: "this was once revealed to me in a dream")