Although Asterisk has many built-in audio files, most often you will want to have custom audio files for prompts for your auto attendants. You can either record them using the phone system itself (often times this can be somewhat confusing), or you can record them with conventional computer recording software and convert them into the proper format.
Most recording software has the ability to record audio, then export it into the proper format that the phone system will use. For Asterisk-based systems, this depends on the CODEC you are using. 90% of the time, the g711 uLAW codec is used, which means the sound file must be an 8kHz, mono, 16-bit audio file.
The best way to convert your audio files for use with Asterisk is to use Digium’s online audio file converter, located here: http://my.digium.com/en/products/ivr/audio-converter/
Instructions are provided on the web page. Once you download the file, you can upload it onto your Asterisk server and use it in your prompts.