There are four Alexa speakers in the house; the original Echo in the living room, an Echo Dot in each of the bathroom and the kitchen, and an Echo Show in the bedroom. All four are in a Multi-Room Music group named "Everywhere" which I seem to recall is automatically created when you have more than one device, and the kitchen and living room speakers are in a group called "Downstairs."
This all seems to work fine for music. There must be some sophisticated going on with the sound in the background, because the synchronisation of all four speakers is spot on which seems remarkable to me over a WiFi TCP/IP network. I never tire of saying "Alex, play If I Fell downstairs" to kick off the Beatles or "Alexa, Diana Krall downstairs" (crawl downstairs, geddit!?) to get songs from Mrs Elvis Costello. Simple pleasures for small minds I suppose.
Alexa is surprisingly good at reading out Kindle books, and though I can get it working on the Echo Show because I can select the book from my library on its screen I have never managed to get any mileage at all out it on the other three devices just by using my voice, never mind getting it to use the Everywhere group. Audible is better with regards to voice control, but it is a bit hit and miss.
I've discovered a work round with the Alexa app on my phone though. Open it and press the play button at the bottom of the screen. if you scroll down you will be (or rather I was) presented with your Kindle and Audible libraries. Swipe through the thumbnails of the books listed and tap the one you want to continue listening to. At this stage you will be presented with a list of all the groups. If I select the "Everywhere" group I can get my fix throughout the house as I am pottering about. BBC sounds doesn't work like this, so I can't get Radio 4, and podcasts are a but tricky as you seem to have to select them from the recently played list, but all in all a significant enhancement of my experience. This is certainly how I will be doing my daily Arabian Nights story from here on in.