HOW I DO IT: I have a Echo dot downstairs named "stereo", it's hooked to three amplifiers with the 3mm stereo cable. Those amps are turned on with a Alexa routine by saying "Alexa turn on the stereos". Then I ask any of my other echo devices to play a song on the stereo (Remember the DOT downstairs is named stereo). The dot named "stereo" plays the music and it comes out of my whole house speakers. I control the volume by saying "Alexa, set the volume on the stereo to 8 (or whatever).
I can also choose to play sound from my PC on those same amps. I have a DPDT relay:
https://smile.amazon.com/gp/product/B00LWX491K/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1that I use to switch between the dot and the PC inputs. It also comes in 12 volt versions. I just cut and stripped the 3.5mm input cables. One input from the DOT, the other from the PC. The output from the relay splits to all three amps.
I use a smart plug I named "sound switcher" to switch a power adapter on or off. It has a 5 volt DC adapter plugged into it. When I say "Alexa turn on sound switcher" it turns on the adapter and powers the relay. The relay then switches inputs (both left and right channel) to the PC. When the adapter is off, the input comes from the DOT by default.
The relay just switches the left and right 3mm cable inputs. I left the ground connected on both all the time.
Works great. I have 6 sets of speakers in my home but if you live around me, you may already know that.
NOTE: I can switch the amps on and off separately. One is named Livingroom. One is named "Shop" and the third is named "Bathroom" but all three can be controled with a group by saying "Alexa turn on the stereos".
NOTE: PC volume is controlled by the PC volume setting.
NOTE: Dot named stereo is always set on mute and drop in is set to off with notifications also off.