Depending on the extension's configuration, Yomitan can sometimes run into issues with downloading audio files while creating Anki cards. This may be due to a permissions issue where Yomitan hasn't been granted access to the sites hosting the audio files.
Check the Site access section of the extension settings pages and grant the extension access to all sites or add the specific audio host URLs.
If a website failes to keep its HTTPS certificate up to date, downloads can fail because the browser flags the connection as insecure. This has happened occasionally for some websites that Yomitan interacts with, and the issue is usually resolved within a day.
This issue is a server-side issue that Yomitan doesn't have control over.
Audio files can be downloaded from remote servers when creating Anki cards, and sometimes these downloads can stall due to server or internet connectivity issues. The Idle download timeout setting on the settings page specifies a time limit for stalled downloads.