Here is a good article by Doug Ward, a University of Kansas professor, who uses iAnnotate to provide verbal grading/feedback on student papers. iAnnotate is the Cadillac of PDF annotations and it syncs with Dropbox. The audio notes are saved with the file. iAnnotate is pricey ($10).
There is another app, PaperPort, that supports text dictation using Dragon speech-recognition software. PaperPort is not fast at importing a PDF document from Dropbox, and you need networking "on" in order to use the text dictation. The handwriting is pretty rough, but typing, dictation and support for PDF make it a good app. The free pricetag doesn't hurt either. Here's a good review.
