It might also be a Windows issue, here are the instructions you can try on Windows 8 and Windows 10.
Windows 8 instructions
Maps Pro uses the text-to-speech feature of Windows 8 so you will need to check if the Windows is well configured. Press the windows key, then start typing "text" (or the equivalent in your language). A setting should come up in the result: "Change text to speech settings":
Click it and it will open the text-to-speech properties dialog window:
There, you can choose the voice and test the result with the "Preview Voice" button. If it works there, it will work in Maps Pro.
If it does not solve the problem, try closing other Windows Store applications. There is only one Windows Store application that can use background audio on Windows 8 so it is possible that some other application such as Skype or Steam took the priority on Maps Pro.
If you closed the application and it still doesn't work, there might be another issue. Some users reported that the voice wouldn't work in Maps Pro, even though the voice preview worked well. The issue is actually on Microsoft's side and you need to change the Windows registry to get the voice to work, see these articles:
Windows 10 instructions
Some users reported that voice didn't work properly on Windows 10. After investigation, it turns out some installations of Windows don't come with a default voice, such as Dutch Windows installs.
To check if a voice is enabled on your system, go to "Region and Language" settings on Windows, then "Speech". If the panel is greyed out, it means that there is no voice installed.
To fix this, you need to manually install the voice on your system. Here are the instructions on how to install a voice manually on your PC:
Maps Pro users reported than German voice worked best with Dutch language
Windows "N" editions instructions
Here is how to get audio working with Windows "N" editions:
How to stream audio when using Windows "N" edition?
Still doesn't work?
If it still doesnt't work, please send an email to firstname.lastname@example.org