Converting audio files such as .aiff or .wav to .mp3 in iTunes is a one-step process, but it takes several steps to setup for the first time.

Here's how to set it up:

1. Open iTunes.
2. Under the iTunes menu at the top, click "Preferences" (mac)
3. Click on the "Advanced" icon section.
4. In the advanced section select the "Importing" tab.
5. Change the "Import Using:" drop down menu to "MP3 Encoder"
6. Make sure the bit rate "Setting:" is at "128kbps" or more.
7. Click "OK"

This might seem counterintuitive, since you are changing the "import" option. But this setting also affects the file conversion in iTunes.

After you have changed this default setting, you can now convert to mp3 by doing the following:

1. Click on the audio file you wish to convert in iTunes
2. Under the "Advanced" menu at the top of the screen, select the "Convert Selection to MP3"

This will create a second file that is in MP3 format. You can drag this file off of your "Now Playing" list in to a folder or onto your desktop. This file is prepared in the correct format for Soundslides to import.

If you do not wish to use iTunes to convert the file, there is another option to use Soundslides to convert to MP3.