Download and install Audacity if you don't have it.
Record the audio via any means comfortable to you, you can use Audacity also but I will not cover that in this post.
After you open the file to Audacity as a new project and adjust the track settings as
Mono
Then select the project to 8000Hz
Then select the track that you want to export and export audio as:
Filename: .wav
Save as Type: Other uncompressed files
Header: WAV (Microsoft)
Encoding: U-Law
That's all, now you can use the file as an Unity prompt!
