An AssetScript defines a file containing JavaScript (technically ECMAScript 3 compliant code) that you can run in the FireBoxRoom. Functions can be set up for specific events such as mouse clicks or on frame updates. Sounds can be played, Objects can be created, have their properties modified, and be destroyed. For further details about this, see this page: http://janusvr.com/docs/build/roomobject/index.html. Also check the "experiment" workspace included with JanusVR which serves as a JS example.
http://janusvr.com/docs/build/assetstag/index.html#AssetScript
#onclick is used for the JS that plays the sound
http://demos.janusxr.org/JavaScript/index.html