A very nice link in, of all places, GERMANY!!
http://www.josef-bayer.de/wales/index.htm.
Has information not only on penillion, but on the Cymanfa Ganu and a lot of history and even current music. Very comprehensive. The site is owned by Josef Bayer and family. He is married to a Welsh lady from Pembrokeshire (West Wales) and they have two daughters. Lots more on the site, but my German's a bit rusty. The section relevant to Wales is, however, in English. Well worth the time.
There are many other sites. Just type in penillion in your favorite search engine.
Unfortunately, I don't know of any samples on the net. Not popular enough to put up I suppose. However, Sain (pronounced sine) has tapes and CDs for sale at their website
http://www.sain.wales.com/english/sainshop/catalogue/cerdddant.htm
Dave Williams