There are different ways to add audio files. These depend on what format you are starting with, whether you want to add images and creative decisions.

For audio only (under 10 mins and under 200Mb) use AudioBoom - http://audioboom.com. You will need a free account. You can record an audio file or upload one - press the red button in the top right hand corner. Files take a while to process, so patience.... You can upload lots of files at once which saves time. You can also add a cover image in AudioBoom which will automatically show when used on the Stories of Change website.

Audacity - http://audacityteam.org - is a great piece of free software that you can use to convert WAV files to MP3. AudioBoom won't allow large WAV files to upload, so you may need to convert them to MP3 files to reduce their size.

You will also need to install a file called Lame - http://lame.buanzo.org - which converts to MP3 format.

For audio with images you can make a movie slide show using iMovie or similar and upload the result to Vimeo - http://vimeo.com or Youtube - http://youtube.com.

The best way to add to all of these is to the Stories of Change website is to copy / paste the embed code which you get as a sharing option.