on iOS lock screen, I can't play the audio

[akio46]

first audio is started to be played, few mins later, lock screen appears.

after first audio is finished, the second audio isn't played.

        const selectedTrack = this.audioProvider.tracks[index].id;
        this.audioProvider.play(selectedTrack);

index is just 1 because it's second track.

this is the log for first track.

Loading track https://d1as0hz72d196v.cloudfront.net/production/audio/1._ESS_5_day_Meditation_Reinvented_NEW_FINAL.mp3 2018-08-12 16:46:18.027071+0800 SOS Method[25961:11184456] Playing track https://d1as0hz72d196v.cloudfront.net/production/audio/1._ESS_5_day_Meditation_Reinvented_NEW_FINAL.mp3

this is the log for second track

Loading track https://d1as0hz72d196v.cloudfront.net/production/audio/01_CALM_and_STRENGTH_5_min_FINAL.mp3

As you can see, I don't see Playing track log for second track.
How can I solve this? thanks