I have been testing the issue you have been experiencing further and have found that the audio is not interrupted if you instead use a Local Video Widget instead of a Web Page Widget. I can also confirm that adding a second video to the Layout also did not interrupt the Radio stream.
You can alternatively create a layout that only contains the Videos you wish to display and a separate Layout for the Radio Stream, making the region that contains the Radio very small, as you did before. You can then schedule the Video Layout as you would normally. For the Radio Layout, you can schedule a separate Event, but change the Event Type to Overlay Layout. This will cause the Videos to display whilst the Radio Overlay Plays independently, allowing you to set a longer duration for the Radio Layout from the Video.
Your internet Radio link is an mpeg stream, which is why it is better suited for use with the Local Video Widget instead of Web Page.
If you continue to experience issues with the Layout, please let me know.
Many Thanks.