Airsonic is using Maven to manage its build process. Any version above 3.3+ should do the job.
If you want to run the testsuite and get a .war is everything went fine,
you this command:
$ mvn clean package
If you don't care about the result of the testsuite, but only
want a .war as quick as possible, you can use this instead:
$ mvn -Dmaven.test.skip=true clean package
Airsonic's source code is hosted on github, who provides a lot of documentation on how to contribute to projects hosted there.
The documentation is hosted here (you can contribute to it here), and aims at being comprehensive. You can also use irc and reddit if you want to discuss or ask questions.