If you have a TV with a feature to turn off the screen, it won't use unnecessary power. Most TVs with that feature reduces the power consumption to a minimum like the Samsung 6470 H-series and many more models. On that model just go to the tools menu with a touch of a button on your remote and select "Screen off". That transforms your TV virtually a smart speaker. Many other manufacturers have similar features.
In the rooms where you have no TV, you can now easily use an Chromecast Audio for $35 and have sound. Sure you can buy an active speaker. But you can also use any speaker together with a small nifty amplifier like the Lepai 808. It's good enough for having decent sound in your kitchen or bathroom (mind the moisture).
Connect a power adapter to your Lepai amplifier, connect the tiny Chromecast Audio with the 3,5mm plug and connect the speakers via the clamps. Here we go. You can also check out what you can do with this little amp to make a boombox. Now think about to add a Chromecast Audio to it and have a mobile boombox for your house where you need it. Just for your inspiration.
However if you want to connect a mono speaker, which is not so bad if you have only little space left, you'll need to get a female stereo to male mono 3,5mm adapter. Do not connect left and right speaker output to one speaker. It will likely kill your amplifier in no time. This little adapter makes sure you hear output from both channels on both channels mixed. As far as I know. it's safe with the Lepai to use only one speaker connected.
Now later on the Chromecast Audio will be updated with multi-room functionality, which means you have a pretty nice and very inexpensive setup in your house. That even guests without access to your WiFi can use.
