OK, let's assume that you have a working audio file that is 20-30 seconds in iTunes. You seem to have it in the wrong format, since the correct file size at 128kb/s should be about 512KB. Out of curiosity, how did you get your snippet down to 20-30 seconds by only using iTunes?
Anyway, Follow these steps:
- Go into the iTunes menu -> Preferences...
- Click on the "General" tab (iTunes 9) or on the "Advanced" tab (<iTunes 8)
- Click on the button "Import Settings" and choose MP3 and Setting: Custom
- In most cases, you won't miss any crucial information if you set the Stereo Bitrate to 64-128, the Sample Rate to 22khz and Channels to Mono
- Back in the main iTunes window, select the 30 second clip and then choose the menu Advanced -> Create mp3 version
- Select the converted mp3 clip, choose File -> Show In Finder and your clip should be ready for upload.
If this doesn't solve your problem, download Audacity
and use that to edit and export your clip. Good luck!