GMail's audio attachments started failing to play with Google Chrome 91.0.4472.77. The console shows:

Refused to load media from 'https://mail.google.com/mail/u/...' because it violates the following Content Security Policy directive: "media-src https://*.googlevideo.com/videoplayback/".

Seems that GMail isn't following the newly activated CSPs. Until Google fixes it, this simple extension resolves the problem.