解决方案

   /**
     * 播放raw目录下文件
     */
    private fun playFromRaw(): MediaPlayer? {
        mediaPlayer = MediaPlayer.create(this, R.raw.test)
        mediaPlayer.start()
        return mediaPlayer
    }

    /**
     * 播放assets目录下文件
     */
    private fun playFromAssets(): MediaPlayer {
        val fd = assets.openFd("test.mp3")
        mediaPlayer = MediaPlayer()
        mediaPlayer.setDataSource(fd.fileDescriptor, fd.startOffset, fd.length)
        mediaPlayer.prepare()
        mediaPlayer.start()
        return mediaPlayer
    }

    /**
     * 播放SD卡上文件
     * 需要动态申请SD读权限
     */
    private fun playFromSdCard(): MediaPlayer {
        val path = "/sdcard/test.mp3"
        mediaPlayer = MediaPlayer()
        mediaPlayer.setDataSource(path)
        mediaPlayer.prepare()
        mediaPlayer.start()
        return mediaPlayer
    }

    /**
     * 播放URL对应文件
     */
    private fun playFromUrl(): MediaPlayer {
        val audioUrl = "https://gitee.com/cxyzy1/audioPlayerDemo/raw/master/test.mp3"     //这里给一个歌曲的网络地址就行了
        val uri = Uri.parse(audioUrl)
        mediaPlayer = MediaPlayer.create(this, uri)
        mediaPlayer.start()
        return mediaPlayer
    }