ac3 is a the raw multi-channel stream normally with 5.1 audio.
multichannel audio on a laserdisc used to be AC3 😉
and the DVD inherited that.
There are many multi-channel audio codecs, and I personally havent ever seen a WAV file with more than 2 channels. prolly thats just the RIFF/WAVE standard, though.
I too got many music DVDs that Id like to listen to in 5.1 over the soundbridge. I havent gotten around to buying a decent sound receiver though.