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