The most common reason for large audio files is that folks recorded things using the best quality (generally much better than the microphone and equipment they were using could produce) and are fretful about reducing it, or they don't want to take the time to reduce it.
There is a bit of a standard out there -- iTunes and Microsoft Windows media player "rip" of a CD. We can assume that what these use to capture and deliver professional high quality recordings is more than sufficient for recordings of lesser production value. Here are dome data points:
- Mariah Carey "Rainbow" album, 55 minutes ==> 38Mbytes, with album art included
- Whitney Houston "Heartbreaker album, 60 minutes ==> 42Mbytes, with album art included
- "Phantom of the Opera" soundtrack, 59 minutes ==> 54Mbytes, including album art included
A file search of one users fairly decent "rip" library could find only a handful of tracks larger than the 16Mbyte limit. These were captures with the default Microsoft settings of 128Kbps.
So the first question to ask yourself regarding large audio files is "Do I really sound so much better than Mariah Carey that I need to use higher quality reproductions than she does?"
It may be a challenge to reduce the size of the file, but it is needed for effective web delivery.