I just got a new G5 a few weeks ago. I am running 10.3.8. I have been trying to purchase some drum and effects loops from a site that uses flash (www.p5audio.com). I can not hear audio playback from the flash play buttons, and when I try to access the shopping cart, when attempting to purchase, I am given the following message: "shopping cart is empty, click here to return to the site." This happens using IE and Safari. I do NOT have this problem on my girlfriend's eMac G4 with 10.3.8. Can someone please help me?
Thanks.

make sure you have the latest version of Flash plugins installed. Also try using Firefox. IE is pretty much worthless these days as Microsoft is no longer creating new versions and have given up on it for the mac and while Safari is nice... it has its shortcomings too.

oh... also make sure there are not any software updates available for your system that have yet to be installed

What is IE?

You should use in priority Safari or Firefox. And on sites that don't behave try the ones between Safari or Firefox which is not you browser of choice, then only try IE.

Now beside that, as Synthetic pointed out, have you updated everything and have you especially repaired permissions using Apple disk utility and restarted?