gain.js

// Create a gain node and set the initial value to 0.5
// that means that volume will be haft of the original.
const gainNode = new GainNode(context, {gain: 0.5});

// Disconnect source before constructing a new graph.
lineInSource.disconnect();

// Connect nodes
lineInSource.connect(gainNode).connect(context.destination);

// Increasing volume.
gainNode.gain.value = 2;