Follow the spec

https://github.com/matrix-org/matrix-doc/blob/travis/msc/voice-messages/proposals/3245-voice-messages.md#unstable-prefix

Author: bmarty

matrix-sdk-android/src/main/java/org/matrix/android/sdk/api/session/room/model/message/MessageAudioContent.kt

@@ -20,6 +20,7 @@ import com.squareup.moshi.Json
 import com.squareup.moshi.JsonClass
 import org.matrix.android.sdk.api.session.events.model.Content
 import org.matrix.android.sdk.api.session.room.model.relation.RelationDefaultContent
+import org.matrix.android.sdk.api.util.JsonDict
 import org.matrix.android.sdk.internal.crypto.model.rest.AudioWaveformInfo
 import org.matrix.android.sdk.internal.crypto.model.rest.EncryptedFileInfo
 
@@ -61,7 +62,7 @@ data class MessageAudioContent(
  /**
  * Indicates that is a voice message.
  */
- @Json(name = "org.matrix.msc2516.voice") val voiceMessageIndicator: Any? = null
+ @Json(name = "org.matrix.msc3245.voice") val voiceMessageIndicator: JsonDict? = null
 ) : MessageWithAttachmentContent {
 
  override val mimeType: String?

matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/session/room/send/LocalEchoEventFactory.kt

@@ -303,7 +303,7 @@ internal class LocalEchoEventFactory @Inject constructor(
  duration = attachment.duration?.toInt(),
  waveform = attachment.waveform
  ),
- voiceMessageIndicator = if (!isVoiceMessage) null else Any()
+ voiceMessageIndicator = if (!isVoiceMessage) null else emptyMap()
  )
  return createMessageEvent(roomId, content)
  }