Core Audio is definitely built into a Mac, and I was just the other day wishing JRiver could encode AAC like XLD and a number of other programs can, for use in my attempts to get a handheld to sync on a Mac. I would be happy with Ogg, but I didn't expect that. I expected AAC would be available, and was surprised it wasn't. I settled on MP3, since it's just for portable use.
If you just want to encode on a Mac, you're better off using XLD and keeping JRiver for music management and playback. XLD can rip using the AccurateRIP database, can convert to ogg and a wide number of other formats (though not Musepack) and unlike JRiver, XLD adheres to traditional Mac design / interface norms. It's the easiest CD ripper / audio converter I've ever used in Mac, Windows, or Linux, and truly "it just works" like Apple's slogan used to say.
(I don't know of any Mac program that converted to MPC.)
Fre:ac is also fairly easy to use, and powerful and versatile, and updates its ogg encoder still which I don't think XLD has done in a long while, but is not Mac-like. It is my go-to on Linux, and it's great, and also like XLD it is free to use.
There is no application of any sort that I actually recommend as unreservedly as XLD. For what it does, it's overall better than anything else when all factors are considered (versatility, speed, ease of use, reliability, UI).