Sound files should be WAV Mono 8000 Hz.
Instructions on how to record audio using the free app Audacity:



1) Connect a headset to your PC.



2) Install the free app Audacity 



Audio settings: 8000 Hz, Mono, WAV.



3) Select 8000 Hz and 'Mono' input:



 



4) Start recording





5) Click file, then export...





6) Select WAV and Save