If you decide to have music in your PSA you have to be cautious. Because your are posting them to your web site or blog you have to make sure you have the legal right to use any music you plan to use in your PSA. Noble purpose or intent is not a legal defense. Remember how the RIAA went after Jib-Jab? You don't want to know what they could do to you.
There are safe places to obtain music. It is vital that you make sure you understand you can and cannot do with the music you are using for your PSA (or any other purpose). Here are a few starting places:
Flashkit has hundreds of music loops and sound effects that are free, free for non-commercial use, or free provided you inform the author of the work how and where the musical loop is to be used. You can preview the music loop before you download it. http://www.flashkit.com
Videomaker Magazine has a download page that links to companies offering free music and musical loops.
Yahoo has a Creative Commons Search for music that can be used, modified or even used for commercial purposes (read the fine print first!) You can type in the kind of music that you are looking for and received a boatload of suggestions. At this time, the search was still in Beta form. Some of my searches pulled up non-CCS type music or education pages that mentioned music but has none for download.
But I did two good places to check out; Royalty Free Music Piano Solos and Fonogeno, which has a page of electronic music such as beats, percussion, and synthetic bass.
If your music is in a format other than .wav you will need to convert it into the correct format. Dubit can only import .wav files.
Audacity is a free and non-adware/spyware audio conversion program. All that means is that you can use it to convert MP3 into .wav sound files. You can also download a copy from Download.com http://www.download.com.
Once you have your sound file in a .wav format, you can insert it into your PSA.
Next - Previewing, File Formats and Distribution Options