I tried other solutions that did not work for me. I could not install a fresh ubuntu iso; nothing shows in log. Be sure to back up your existing virtual machine, and understand these steps before proceeding at your own risk.
Finally, I did
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brew cask uninstall virtualbox virtualbox-extension-pack
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brew cask install virtualbox-beta
The new virtualbox-beta prompts you to permit sound in OSX security settings. Once you allow sound, and finish your install, you can uninstall the beta, then re-install virtualbox.