Mp3 upload dark / light bar:
Youtube video full size. Careful: if video has ads, they will be visible! It may be best to upload a custom video to make sure there is no ad monetization. Additionally, video suggestions will be shown when video is over.
Youtube video smallest size. All above comments about ads stay true.
Soundcloud player in medium size (to allow guests to see where/how to pause music). Soundcloud has no ads but song library is mainly indie productions. No autoplay here so it doesn’t conflict with youtube video above.