Your clerk is lying to you (and making-up regulations that do not exist). I can mail a Britney Spears CD, or a Titanic DVD, via media mail even though these items are published by commercial companies. I've done it many, many times in many, many post offices ranging from New England to Michigan to Oklahoma.
Next time, try a different post office that DOES allow CDs/DVDs to be mailed as media.
Also of note: "Sound recordings may include incidental announcements [advertisements] of recordings and guides or scripts prepared solely for use with such recordings." -
http://pe.usps.gov/text/qsg300/Q370.htm
Where is your post office located? I'd like to go check it out myself (I do a LOT of travel & eventually get to everywhere). I'd like to talk to these clerks face-to-face to see if they refuse to let me ship a Titanic or Britney CD/DVD via media mail.
Book Rate and Media Mail are the same thing. (Media is the formal term; book rate is the slang.) If you think I'm wrong, please provide the usps.gov link that describes "book rate" as distinct from media mail. Thanks.
Bottom Line:
You have a lousy, idiotic person running your post office. I've shipped from many, many different offices all across the U.S. and every one of them allowed me to ship CDs/DVD regardless of the content of those recordings (music, video, or games).
Repeat: Where is your USPS office located? I want to visit them.