The current version of Audacity (2.3.0) is not supported by macOS 10.14.4 beta.
The next version of Audacity for Mac will be a 64-bit application. Release candidates for Audacity 2.3.1 (64-bit) are expected to be available soon, and will be announced on this forum.
Update: 64-bit Audacity 2.3.1 is now available.