If you follow the link that @JDRoberts posted it will tell you about LANDroid. There is another program in there also, Big Talker. I started out using it but changed over my speech requirements to Alexa Helper and Rule Machine.
Basically, based on what you’ve said you want, this is how I would start it out…
Install LANDroid into ST and onto your cell phone. If you have an older phone laying around in a drawer, even better.
You connect the phone to Alexa via Bluetooth. You will also have your phone connected to your network. Don’t worry, it only works when your phone is connected to the network. (I think)
LANDroid is running in the background on the phone.
You set up a rule in Rule Machine that when vibration is detected on your multi sensor it sends a notification via speech. You choose your device, which shows up as a text to speech device.
When someone knocks, the multi-sensor goes active, the rule activates and Alexa speaks your message for you. There is a slight delay because it is a cloud - wi-fi process. But not usually more than a couple of seconds.
It does not speak in the normal Alexa voice. It is slightly different because of the TTS.
If you are playing music or something, Alexa will pause that and play the message. I found that building in a slight delay of about one second keeps Alexa from talking over the currently playing stuff. You can do that by adding a bunch of periods (…) right before the text. Just experiment.
I did migrate away from Alexa after a while. I have a small speaker that I was able to hide behind some stuff. It is plugged into the old phone which is always on wi-fi. So, when my front door opens, or the garage door is closed, or whatever it plays there.
What is really neat about it all. I have 3 Echoes in my house. I can connect a separate instance of LANDroid on multiple devices, connect them to each Echo and I can choose which one the messages play on.
Such as, I currently have a temperature report. I ask Alexa to turn on VS and she my thermostat is polled for the current temp and what it is set to. It plays over the speakers. Well, if I’m upstairs and I want to know something I won’t want the message playing downstairs. So, I set up multiple VS for the same Report Scenario. I just call them by whatever when I tell her to turn it on. If I’m upstairs she just plays it up there. If I’m in the garage, she will play it there only. Or, I can have her play different things through the whole house, or I can turn on a VS that excludes the speaker in the bedroom when the baby is sleeping.
ST, Alexa, Alexa Helper, and Rule Machine have made my home amazing!