Control + Enter updates the code to run the synthesizer with

Control + Space plays/pauses the audio synthesizer: if your keyboard has a media key, you can use that to the same effect

To emit audio you need to export a function (and nothing else) which takes the time in seconds since the start of the tune and which returns a number between -1 and 1

Below is a working example for reference: t is the current time in seconds, while i is the 0-based samples counter 

export function f(t, i) {
 const tau = 2 * Math.PI
 return Math.sin(500 * tau * t)
}