在播放第二个声音时，先暂停第一个声音，等第二个声音播放完毕后，再恢复第一个声音的播放。

具体代码如下：

import AVFoundation

class ViewController: UIViewController, AVAudioPlayerDelegate {

    var player1: AVAudioPlayer!
    var player2: AVAudioPlayer!

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 创建player1和player2
        let path1 = Bundle.main.path(forResource: "sound1", ofType: "mp3")
        let url1 = NSURL.fileURL(withPath: path1!)
        let path2 = Bundle.main.path(forResource: "sound2", ofType: "mp3")
        let url2 = NSURL.fileURL(withPath: path2!)

        do {
            player1 = try AVAudioPlayer(contentsOf: url1)
            player1.delegate = self
            player1.prepareToPlay()

            player2 = try AVAudioPlayer(contentsOf: url2)
            player2.delegate = self
            player2.prepareToPlay()

        } catch let error as NSError {
            print(error.description)
        }
    }

    func playSound1() {
        player1.play()
    }

    func playSound2() {
        player1.pause()
        player2.play()
    }

    func audioPlayerDidFinishPlaying(_ player: AVAudioPlayer, successfully flag: Bool) {
        if player == player2 {
            player1.play()
        }
    }
}