This is currently one of the best site for information about Manx Gaelic. The site includes sound samples of native speakers in .wav format; my favorite is the one about the pig.
The Centre researches and publishes in every area of Manx studies; many of their materials are available freely online, including their journal Laare-Studeyrys Manninage in Manx, and current archaeological excavation reports (in English).
This site from Dr. Stephen Miller contains publications in, and about, Manx Gaelic. Much of the site can be downloaded in Adobe .pdf format as well as in ordinary text.