PlayerActivity.kt

   override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_player)
        val mediaSource = extractMediaSourceFromUri(Uri.parse("asset:///heart_attack.mp3"))
        val exoPlayer = ExoPlayerFactory.newSimpleInstance(
            baseContext, DefaultRenderersFactory(baseContext)
            , DefaultTrackSelector(),
            DefaultLoadControl()
        )
        exoPlayer.apply {
            // AudioAttributes here from exoplayer package !!!
            val attr = AudioAttributes.Builder().setUsage(C.USAGE_MEDIA)
                .setContentType(C.CONTENT_TYPE_MUSIC)
                .build()
            // In 2.9.X you don't need to manually handle audio focus :D
            setAudioAttributes(attr, true)
            prepare(mediaSource)
            // THAT IS ALL YOU NEED 
            playWhenReady = true
        }
    }