There are two different custom device type handlers available for the Aeon doorbell and only one of them can handle voice files. And it’s all a bit fiddly. If you want to continue to work with that device, you certainly can: talk to the people in the following thread and they should be able to help you work through it step-by-step. ( this is a clickable link)
That said, as of late 2018 the easiest way to get this function, and also less expensive, is just to get an Amazon Echo Dot. So if you are still within the return period, You might consider returning the Aeon device and getting an Echo Device instead, unless you need the physical button as well.
Amazon has their own routines which you set up in the echo app, and they already have the ability to trigger off of a contact sensor that is connected to SmartThings being opened. And they let you create a custom phrase for the echo to say when that contact sensor is opened, by using their routines.
https://www.amazon.com/gp/help/customer/display.html?nodeId=G202200080
So that’s all you have to do. No custom code required. When the front door sensor opens, have echo say “the front door is open.“ When the back door opens, use a different Amazon routine to have echo say “the back door is open.“
You can even use the “notifications“ feature in the Amazon routine to have multiple echoes say the same thing at the same time.
So it works very well and it costs less than the Aeotec Doorbell and it’s easier to set up. 
This will work with any echo device, so if you already have a regular echo or a show you could use that instead of getting a new dot. It’s just that the dot is the least expensive.
And of course you get all of the other echo features as well. But the Aeon has the physical button, too. Choice is good.