Fix build when run on pre-macOS Monterey

[jwoglom]

When run on macOS versions before Monterey (12.0), the kAudioObjectPropertyElementMain constant does not exist, resulting in a failed build. This is equivalent to the deprecated kAudioObjectPropertyElementMaster constant, which does work. This PR adds a #ifndef to use the old constant pre-Monterey which fixes builds on older macOS versions (e.g. Catalina and Big Sur).