Long audio file stops playing when phone goes to sleep

I am putting together a survey which requires participants to listen to a 20 minute audio file (uploaded as MP3) on their mobile phone. On some phones, the entire audio file plays just fine (iPhone 11) but on others, the audio stops a few minutes after the screen goes to sleep (iPhone 7, Pixel 3).
I am wondering if anyone has any idea how to circumvent this. I imagine it has something to do with the audio file being embedded in a web page vs. playing in a media player.
Thanks in advance for your help!