Just wanted to add that Nick Jr. has a show called The Wonder Pets, which is presented as an Opera. It’s really fun.
I don’t know anything about art songs, but when I was first dipping my ears into the opera ocean, I picked up a couple of CDs of “best loved arias”, that sort of thing.
My two first operas to buy were Carmen and The Magic Flute. Both those have wonderful arias that are very accessible to new listeners. (i.e. - catchy as hell!)
I beg to differ, and will shoot down your statement with two words: Verdi’s Otello. It has an AM far superior to Schubert’s.
My suggestion is to watch it on video first. The von Karajin version, please. Yes, it’s a little 70s looking, but IMHO the best directed version. That way you can get to know the plot. Then you can appreciate a CD version all the more.
How does that shoot my statement down? I was talking about the specific “Ave Maria” (by Schubert) that Morath had heard, you silly person. 
It’s really more like La Traviata set with the characters from La Boheme.
I just wanted to pop in and mention that the Schubert Ave Maria is not, in the original version, the traditional Latin prayer, rather it is an excerpt (Ellen’s third prayer, I believe) from Walter Scott’s Lady of the Lake (translated into German). Morath, I suspect that if you heard it in German, this is the version you heard. In my not so humble opinion, this is how it should always be done. If you really need a Latin Ave Maria, there are plenty of others to choose from.