The audio itself might be input from a microphone or other audio sampling device, or might be received through a RTCPeerConnection, among other possible options.
A MediaStreamTrackAudioSourceNode has no inputs and exactly one output, and is created using the AudioContext.createMediaStreamTrackSource() method. This interface is similar to MediaStreamAudioSourceNode, except it lets you specifically state the track to use, rather than assuming the first audio track on a stream.
68Firefox 68 implements the updated standard's definition of the "first" audio track; now the first track is the one whose ID comes first lexicographically.
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68Firefox 68 implements the updated standard's definition of the "first" audio track; now the first track is the one whose ID comes first lexicographically.