I'm going to assume he means the "Audience" total in Mediabase for a certain song. In Mediabase, it's listed in Millions... so if a song has an "Aud" of 0.61, then it was heard by approximately 610,000 people for the week. (Of course, with people tuned in multiple times over a week for periods of time, I'll also assume that the 610,000 includes many people who heard it multiple times.)
Based off of ratings numbers across dayparts, I believe Mediabase takes how many times a song was played in each daypart, and how many people on average listen in that daypart. Add it all up, and you have a general "Audience" total.
Of course I could be COMPLETELY wrong. But that's how I've interpreted it.