Are these audio files then like the free ones available on desiringgod.org? I was under the impression they were the manuscripts since they are a logos resource.

These are text files, not audio files, and they are searchable. Like other text resources, you can do specific word and phrase searches in the "basic" tab of your Logos search, by selecting "Sermons from John Piper" in the search "in" options. It will then show a list of every occurrence of that word or phrase in the resource. You can also search using, e.g.,"mytag:piper", if you set up a tag for the resources. This comes in three sets (1980 - 1989, 1990 - 1999, and 2000 - 2014). If you have any other questions, let me know, and I'll try to give you an answer.