The Android operating system versions 5.0 and greater have known bugs related to memory management (see here, here, and here). Over time, available memory is reduced, causing unexpected behavior. One of the common consequences of this bug is that the operating system eventually shuts down any audio that is playing in order to conserve memory. If you use any apps that have their own memory management bugs, including apps that run in the background, this can exacerbate the bug in the operating system. Only a small percentage of our users appear to ever experience this issue (and we have never been able to reproduce it on our testing devices, nor have the app testing companies we've hired).
If you are experiencing this issue, rebooting should correct it. However, it can recur as the available memory is again reduced as a result of the bug. Deleting problematic apps that run in the background will also help. See also this article with instructions on identifying problematic apps on your device.
Interrupted audio can also occur when your device overheats, which can be caused by placing it under your pillow while listening.