[haertig]

At the beginning of your post you mentioned that this affects many different connected Bluetooth devices (speakers). The common item was apparently your phone that was playing the audiobook. This would lead me to believe that it was something on your phone that was causing the problem. But later in your post you mention that you do not see the problem on several other bluetooth devices.

What you need to do is isolate if the problem is in your phone, or in the connected bluetooth devices. One way to begin this investigation is to ask a friend to use their phone to try to play audiobooks on your problem bluetooth devices.

You are trying to see if the problem is following your devices, or your phone. Note that you should only change one parameter at a time, because if you change multiple things between tests, you don't know which thing caused the problem. So when testing with your friends phone it would be best to install the exact same audiobook player app and play the exact same audiobook. This implies that if your phone is Android, find a friend with a phone that runs Android too.

You also need to identify all the components that you want to isolate (test). i.e., what could potentially be the cause of the problem? A few things come immediately to mind: it could be your bluetooth speaker(s), it could be your phone, it could be the app you're running on your phone, it could be the specific audiobook you are playing, it could be a power saving setting as you mentioned. Those are off the top of my head, there could be other possible culprits. You should try to identify all the potential culprits and then do some testing my changing only one possible culprit at a time. Don't take short cuts and skip tests, and don't rule out things based on a hunch, don't indict something based on a hunch - you must test and prove/disprove specific things.

This is basic scientific "design of experiments".

FWIW, I have never seen a power saving setting, even an aggressive one, that would cut power in the "fraction of a second" that you are reporting. And you say you are seeing this on multiple different Bluetooth devices. As I said above, you don't want to rule this out without testing to prove/disprove, but I propose that you do not have multiple devices (I assume different brands, models, etc.) that were all designed by brain-damaged engineers who kick in power saving in a fraction of a second. That makes absolutely zero engineering sense.

You also mentioned "recently" in your post. I'm assuming this means that your same problem devices used to work OK but now they don't. If this is indeed true, then that makes the aggressive power saving theory even more unbelievable. Multiple different devices all went bad at the same time? That's not terribly plausible.

If I were to hazard an engineering "guess", I'd say this is probably an issue with your phone. Or the app you are using on the phone. Or something in the OS of the phone.
Making it harder to diagnose is that it is most likely an intermittent problem. e.g., the Echo devices that you say are working fine today may not be working fine tomorrow. You just haven't seen that happen ... yet. It may work for a while, then not work for a while, and it's somewhat random bad luck for a device to be connected when it stops working, since that device then gets blamed.

I believe you may be chasing intermittentcy at this point, and that is preventing you from getting reliable test results. Note that with intermittent problems, testing something only once may not be enough. It is possible, quite likely actually, that you may need to test and re-test the same potential culprit over and over to get a reliable result. I would first try re-running all the tests you've done thus far from a friends phone. My hunch is that this testing with a different phone might shed a little light on the subject. But you never know, the testing is still required - my hunch is only a hunch, it holds no value when compared to actual testing.