SQL Audit and AlwaysOn Replicas
Reference: SQL Server Audit (Database Engine)


  If we enable audit on the main database, will it automatically get
  replicated to our replica?


No it will not automatically get replicated. 

The Database Audit Specification object also belongs to a SQL Server audit. 

The Server Audit Specification object belongs to an audit. You can create one server audit specification per audit, because both are created at the SQL Server instance scope.