i guess a formula would be
the size of the file (s) x the number of downloads = data transfer usage ... for those files alone
You'll have to estimate the expected amount of traffic and possible downloads if this is a brand new site so it might take a month or so to fine tune it to what will be needed plus you'll want room to grow. If the group is good they'll become more popular over time.
plus i'm sure you'll have some images and other graphics as well as the forum, the blogs and so forth will require some data transfer and how much will also depend on the amount of traffic you expect.
All in all, given what the friend wants to do with the site, you might as well look for a very large hosting plan because the clips alone will eat up allotted data transfer quickly. mp3 format can be compressed and edited for the smallest possible file sizes but still won't be very small. He might want to think about cutting those clips to 30 seconds or below as well. Most songs are from 120- 180 seconds (2-3 minutes) long. half that would be giving away a whole lot of song, you know? He really wants to give them just enough to want to hear more plus save some of his usage.
