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