Here's a thread that discusses a cheap way to decode the AC-3 audio (if you don't already have a home theatre system that will do it for you). You can find those Sonic Voom units on eBay for about $20, shipped in the U.S.
I would assume it was brought up in one or the other of those threads, but . . .
I do believe all the FTA hi def receivers will decode the AC3 and put it out the RCA audio leads.
It seems a common feature mentioned in all the reviews I've read.
... or you can take out digital audio on its own RCA or optical connector.
I use BA-735s for PCM (uncompressed) digital sound from a coax digital output, but they won't work on Dolby Digital (compressed) signals. If you tune to an AC-3-only channel, all you'll get is high-pitched shrieking. They don't seem to have an onboard Dolby Digital decoder.