Sound cuts out periodically with my Audiobook.

I have a iPhone 6 running iOS 9.3.1 (also seen the problem with other versions of ioS 9) where on occasion I would be listening to an audiobook through the iBooks app and the sound would go out at certain spots in the book. The audiobook is still playing and I see the time counter still advancing. The sound would eventually return after times ranging from a few seconds to a couple of minutes. When I try to rewind back to the silent spot, it's still silent.

The problem occurs with both using the using my Bluetooth headset and the iPhone speakers.

I don't have any other apps running in the background.

I've stopped the app and restarted the app and it still has the silent portions in the audiobook track.

I've tried changing to another book and changing back and the problem remains.

I've tried jumping to a different chapter and going back to the original problem area and the problem remains.

The only times where I've seen the problem resolve itself is if I close the app and reopen it after a couple of hours or when I rebooted it.

Has anybody else encountered this problem? Or better yet, encountered it and figured out how to avoid it.

Thank in advance.