navigator.getUserMedia() is deprecated. Don't use it in any new projects. However, you can use MediaDevices.getUserMedia() which is experimental. You can specify {video:true, audio:true} as following to record both audio and video.
navigator.mediaDevices.getUserMedia({video:true, audio:true})
.then(function(mediaStream) { ... })
.catch(function(error) { ... })
This way you don't have to merge two streams into one.