With the release of Mojave, Apple has now increased it's security to disable the use of the microphone unless expressed permission is given to an app. You may receive an error message in Studio One stating that "Studio One" would like to access the microphone." To resolve the issue, please go into your System Preferences>Security & Privacy>Privacy and add Studio One to the list of apps that are allowed to use the microphone. Below is a screenshot of where this setting can be found.
For more information on the new features in Mojave, please take a look at the following page from Apple. https://www.apple.com/macos/mojave/