Adjusted Bounce Rate Query

I have a page on a website with embedded live audio, so users go there and sit there listening for up to 90 minutes, but Google Analytics never records a sessions longer than 30 minutes.

I have adjusted bounce rate set to 60 seconds, but “Event 60 seconds on adjusted bounce rate” only appears in Google Analytics sporadically, e.g. after 8 mins, 21 mins, or some other random interval.

What could be going wrong?

Thank you.