There are a lot of different ways, technically speaking, but the simple explaination is to create a WAV file, then convert it to MP3. So now I can read and make these things. So I start collecting all the parts and find pieces missing here and there throughout the postings. Of all of the catagories of groups in the alt. The reasons are fairly simple.
Binary groups are high traffic.
When an ISP gets into a space crunch the binary groups are the first to be sacrificed. Also, alt being what it is, new groups don't get evenly created through all sites. Some will honor newgroup messages, some will ignore them. This group is fairly new and we're still fighting to get better propogation but it takes time. That is, post a note in the group asking for the missing parts to be reposted for you.