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

1) Go to

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