Answer
Start an audio attachment or narration inside a private or People chat. Play, Pause and seeking remain in the message. Returning to it shows the current position. Closing the optional Narration reader returns to the same session.
There is no floating player or background-audio setting. Leaving the chat, switching chats or signing out stops audio; returning does not restart it. Starting another audio item pauses the previous one at its position. Starting a recording also pauses playback. Temporary menus and typing do not stop it.
System media controls show the displayed chat title and Hashchats, which can be visible on the lock screen. Use Play, Pause, Stop and seeking where supported. Android also shows a media notification. After a 10-minute pause on Android, resume from the audio message at its retained position.
Phone and desktop browsers remain subject to browser background restrictions. Composer previews and media-library players do not gain this chat playback behavior.