It is not against their TOS if you do not set any keywords with the ppp ads. And BTW, google is starting to make Audio also...going to be a subsection of adwords. IDK though...PPPads has over 300 Millions websites with their code implemented in it...PPP and Google are gonna be the giants in Audio advertising...
if you want to sign up for PPPAds (free...like adsense is), go to this informative site by ppp:
www.AboutPPPAds.com
There you can watch the video that tells you the fundamentals of ppp.
Thanks for reading...