If you have done everything suggested there's one last thing to check. RF signal strength.
If you have more than one Echo and a friend to help this is easy:
- Leave the [problem] Echo you're troubleshooting in it's location where it is experiencing the problem.
- Place your 2nd device in close proximity to your wireless router.
- Have one device drop in on the other (doesn't matter which initiates as long as you get the connection established).
- You listen at the 2nd device location while your friend talks normally.
- If the audio sounds horrible you can verify it is a signal strength problem by bringing the 1st device closer to your wireless router and retesting. Preferably in another room to avoid confusion and that nasty feedback noise.
You verify it is definitively a signal strength problem if audio of your friend becomes crystal clear on the 2nd device with step 5.
If you have only one Echo that limits your options to test unless you want to pay a professional to come measure signal strength.
- With one echo you can relocate your echo close to your wireless router and see if your problem goes away. If your problem is resolved then you probably have determined it's signal strength. You decide how long to test as well as other variables that impact the probability of your final determination's accuracy.
- Alternately, you can get a long cable and move your wireless router close to your Echo. This may make testing easier but requires you obtain the necessary cable. If you already have a cable that will let you relocate your wireless router this is the option I would suggest.
Signal strength can impact streaming. From my experience drops in signal strength can cause service drops that become unpredictable. I have seen Alexa state the name of a song (when no audio was being output) she thought was being played but actually wasn't because the stream had failed with the likely candidate causing the failure being signal strength.
The Alexa App
should supply a signal strength meter given the information is available within the device. Sadly, there isn't any help here and a signal strength meter could identify this problem during setup rather than forcing users like you through this frustrating experience.
One additional thought: You can use the signal meter built into your smart phone. Just remember it's more of an idiot-guage and you still have to do the above tests and compare results to your phone's meter to derive any value--unless it displays 1 bar or less next to your problem Echo location which could make things somewhat obvious.
Good Luck!