Bloc son : correction a11y (#768)

layouts/partials/blocks/templates/sound.html

 {{- $block := .block -}}
 {{- $block_class := partial "GetBlockClass" .block -}}
 {{- $block_index := .index -}}
+{{- $described_by_id := "" -}}
 
 {{- with .block.data -}}
   <div class="{{ $block_class }}">
@@ -20,11 +21,13 @@
           {{ end }}
 
           {{ if .title }}
-            <p>{{- partial "PrepareHTML" .title -}}</p>
+            {{ $described_by_id = printf "block-sound-title-%d" $block_index }}
+            <p aria-hidden="true" id="{{- $described_by_id -}}">{{- partial "PrepareHTML" .title -}}</p>
           {{ end }}
 
           {{ partial "commons/transcription" ( dict
             "block_index" $block_index
+            "described_by" $described_by_id
             "transcription" .transcription
           ) }}
         </div>

layouts/partials/commons/transcription.html

 {{ if (partial "GetTextFromHTML" .transcription) }}
   {{ $block_index := .block_index }}
-  {{ $id := printf "block-%d-transcription" $block_index}}
+  {{ $id := printf "block-%d-transcription" $block_index }}
   {{ $title := .title | default ( i18n "commons.accessibility.transcription" ) }}
   <div class="transcription">
     <details id="{{- $id -}}">
-      <summary aria-controls="{{- $id -}}" aria-expanded="false">{{ $title }}</summary>
+      <summary 
+        aria-controls="{{- $id -}}" 
+        aria-expanded="false" 
+        {{ with .described_by -}}aria-describedby="{{ . }}"{{- end }}>
+        {{ $title }}
+      </summary>
       {{- $transcription := partial "PrepareHTML" .transcription -}}
       {{ safeHTML $transcription }}
     </details>