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.