To build the voice prompts you need to download the build tools
https://www.opengd77.com/downloads/Voice_Prompts/tools
You also need Python installed on your computer
On windows you can use the build_all.bat file to do a complete build.
Note. You must have an opengd77 radio connected to your PC and turned on, because the scripts need to use the radio to compress the data into AMBE format. AMBE has IP issues and hence AFIK there is no AMBE compressor program that runs natively on either Windows or Linux or Mac etc