Wanted to follow up with this after our internal discussion. I am sorry that I do not have good news as there will not be a plan to address the permission aspect of the mp3 audio embed. It will behave as our other embeds do for videos, images, etc... If you wish to get around this for now in topics, you could zip the mp3 file up so that the audio embedding would not happen. (This feature may disappear in the future, however.)
Overall, in our software, if you wish to have complete file-level access over attachments for member groups, using the Downloads application is advised. You can then copy the Download File's URL and paste it in your topic or have Downloads create topics for you to engage discussion if that is your goal.