diff --git a/client/html/post_upload_row.tpl b/client/html/post_upload_row.tpl
index a1ea46bb..c7cc5dba 100644
--- a/client/html/post_upload_row.tpl
+++ b/client/html/post_upload_row.tpl
@@ -64,16 +64,6 @@
                         }) %>
                     </div>
                 <% } %>
-
-                <% if (['video'].includes(ctx.uploadable.type)) { %>
-                    <div class='loop-video'>
-                        <%= ctx.makeCheckbox({
-                            text: 'Loop video',
-                            name: 'loop-video',
-                            checked: ctx.uploadable.flags.includes('loop'),
-                        }) %>
-                    </div>
-                <% } %>
             </div>
 
             <div class='messages'></div>
diff --git a/client/js/views/post_upload_view.js b/client/js/views/post_upload_view.js
index 3ccf7f35..f1d465fc 100644
--- a/client/js/views/post_upload_view.js
+++ b/client/js/views/post_upload_view.js
@@ -49,12 +49,6 @@ class Uploadable extends events.EventTarget {
     get name() {
         throw new Error('Not implemented');
     }
-
-    _initComplete() {
-        if (['video'].includes(this.type)) {
-            this.flags.push('loop');
-        }
-    }
 }
 
 class File extends Uploadable {
@@ -74,7 +68,6 @@ class File extends Uploadable {
                     new CustomEvent('finish', {detail: {uploadable: this}}));
             });
         }
-        this._initComplete();
     }
 
     destroy() {
@@ -105,7 +98,6 @@ class Url extends Uploadable {
         super();
         this.url = url;
         this.dispatchEvent(new CustomEvent('finish'));
-        this._initComplete();
     }
 
     get mimeType() {
@@ -290,11 +282,6 @@ class PostUploadView extends events.EventTarget {
             uploadable.anonymous = true;
         }
 
-        uploadable.flags = [];
-        if (rowNode.querySelector('.loop-video input:checked')) {
-            uploadable.flags.push('loop');
-        }
-
         uploadable.tags = [];
         uploadable.relations = [];
         for (let [i, lookalike] of uploadable.lookalikes.entries()) {
diff --git a/doc/API.md b/doc/API.md
index c23f0454..6a733f75 100644
--- a/doc/API.md
+++ b/doc/API.md
@@ -843,12 +843,13 @@ data.
     automatically created. Tags created automatically have no implications, no
     suggestions, one name and their category is set to the first tag category
     found. Safety must be any of `"safe"`, `"sketchy"` or `"unsafe"`. Relations
-    must contain valid post IDs. `<flag>` currently can be only `"loop"` to
-    enable looping for video posts. Sending empty `thumbnail` will cause the
-    post to use default thumbnail. If `anonymous` is set to truthy value, the
-    uploader name won't be recorded (privilege verification still applies; it's
-    possible to disallow anonymous uploads completely from config.) For details
-    how to pass `content` and `thumbnail`, see [file uploads](#file-uploads).
+    must contain valid post IDs. If `<flag>` is omitted, they will be defined
+    by default (`"loop"` will be set for all video posts, and `"sound"` will be
+    auto-detected). Sending empty `thumbnail` will cause the post to use default
+    thumbnail. If `anonymous` is set to truthy value, the uploader name won't be
+    recorded (privilege verification still applies; it's possible to disallow
+    anonymous uploads completely from config.) For details on how to pass `content`
+    and `thumbnail`, see [file uploads](#file-uploads).
 
 ## Updating post
 - **Request**
@@ -892,9 +893,9 @@ data.
     automatically created. Tags created automatically have no implications, no
     suggestions, one name and their category is set to the first tag category
     found. Safety must be any of `"safe"`, `"sketchy"` or `"unsafe"`. Relations
-    must contain valid post IDs. `<flag>` currently can be only `"loop"` to
-    enable looping for video posts. Sending empty `thumbnail` will reset the
-    post thumbnail to default. For details how to pass `content` and
+    must contain valid post IDs. `<flag>` can be either `"loop"` to enable looping
+    for video posts or `"sound"` to indicate sound. Sending empty `thumbnail` will
+    reset the post thumbnail to default. For details how to pass `content` and
     `thumbnail`, see [file uploads](#file-uploads). All fields except the
     [`version`](#versioning) are optional - update concerns only provided
     fields.