This is what I ended up with. I think it works pretty well, but I don't really have a basis to judge. I'll have to wait until the end of next month to be able to compare figure with past stats.
This is using logparser to run sql statements on the IIS logs. I had to enable logging of sc-bytes in IIS before it would work.
Note the %music%.mp3. The logs will show /music/BlehBlehBleh.mp3, but I wanted to make sure that anything with "music" and ".mp3" would show up.
Also note the sc-byte >= 1377915. That's 30% of the average filesize of all the music mp3s on that server.
"C:\Program Files (x86)\Log Parser 2.2\LogParser.exe" "SELECT COUNT(cs-uri-stem) FROM c:\inetpub\logs\logfiles\w3svc1\u_ex1104*.log WHERE sc-status=200 AND cs-uri-stem LIKE '%music%.mp3' AND sc-bytes >= 1377915" -i:W3C -o:W3C