Radio world is completely different than podcasting. Radio stations track and report their song usage and pay fees based on that.
But for Podcasting - even if you want to get the licenses to play music and do it legally - you can't - All the needed licenses do not exist. You could pay BMI, SESAC and ASCAP for licenses. And also pay Harry Fox Agency $0.091 per song <5 minutes per download - and besided being broke - you could still get sued.
Online radio has the Sound Exchange license they pay to allow them to "stream" music. But that does not cover and will never cover podcasts - as podcasts are downloads and in the eyes of the law (DMCA) a download is a mechanical copy.
So again - unless the music is specifically "Podsafe" do NOT use it.
Also mashups - they are just as illegal and forbidden as the songs by themselves. Actually Mashups get you into more issues as there is now multiple songs whose copyright you are violating.
Regards,
Rob W
podCast411