怎么样混合背景音乐？

[G-Singularity]

// 1. Layer 1
var url = Bundle.main.url(forResource: "video1", withExtension: "MOV")
var asset = AVAsset(url: url!)
var source = AVAssetSource(asset: asset)
source.selectedTimeRange = CMTimeRange(start: CMTime.zero, duration: asset.duration)
var timeRange = source.selectedTimeRange
let renderLayer1 = RenderLayer(timeRange: timeRange, source: source)
RenderLayer同样适用音频资源么？有没有demo展示一下？

[G-Singularity]

混合背景音乐后，视频不显示了，需要设置什么参数么？