Sound files should be WAV Mono 8000 Hz.
Instructions on how to record audio using the Devhex online recording tool:



1) Go to https://www.devhex.org/audio.php


2) Click on 'Recorder':



3) Click on the recording icon and start recording:



4) Click on the recording icon again when finished:



5) Select 8000 Hz, mono as audio format: 


 6) Click on 'Process file': 



7) Click on 'Download': 





This tool can only be used for recording audio files with a duration less than 1 minute. If you need to record an audio file longer than 1 minute, use Audicity