I may have found a workaround. One day our admininstrative assistant created a post with her WordPress account and it had audio! I tried creating another admin account for myself and couldn’t create audio. I then created an editor role account and Polly worked for me! Our sysadmin cannot find the reason for this after looking at logs why it works for some accounts and not others. Try it out and let me know if it works for you, as an interim fix.
