So I just did some digging and the variable is not actually exposed within blueprints or the editor and can only be set within code. You will need to set this option for the sounds you are currently playing before loading a level, and then set bIgnoreForFlushing to true. I have updated the bug report to reflect the necessary changes.
You can also head to our recently announced Public Bug Tracker and vote for this issue. Let me know if you have further questions or need additional assistance.
Cheers,