I'm not sure of any ways to send it through your Mac, but you could get a simple phono cable (correct word?) and plug it into the headphone jack on the iPad and run it to your audio in on your stereo. Although you may need to run the audio in through a cable that has the red-white connectors for your stereo...
Man, can someone help w/ the proper names of these cables? Way too lazy to look them up right now.
(But a phono is what I use when plugging my iPad into my speakers)
If you want to play music on a Hi Fi while the iPad is charging:
The Apple A/V cables (either with component or composite) can be used to connect the 30pin iPod dock to a HiFi via the RC audio jacks, and then you can charge at the same time via the USB connection.
(and you also will have a cable to hook it up to a T.V if you like at some point)
I beleive the iPad dock has an 3.5 mm output jack that you could get a 3.5mm jack to 2 RCA jacks to plug into your Hi Fi, which would range from $2.00 to $100.00 depending on how much you beleive you should spend.
If you want to play the sound through the computer I guess you coud just use a cable with a 3.5 mm jack at each end, but then why not just play the music directly from the iTunes on the comptuer?