Dell U2410 + ATI Radeon HD4870 = no sound via displayport

Hello!

I have problem with DELL U2410 and ATI Radeon HD4870 on W7. When I have connected both with DP cable, (which was provided by DELL with U2410) I got video without sound. I have"ATI DP output" in Windows 7 sound options, but i can't make it default sound device 'cause, according to W7, it's "unplugged". I already asked on ATI techsupport forum and guy from support have told me, that HD4xxx doesn't support DP audio, but he wasn't sure. In next reply some guy write, that he has the same problem with hd5xxxx ( which does support DP audio for sure)  and DELL 2408WFP. That made me think it's maybe not fault of graphic cards but displays? Are they requirer some additional efforts to get audio from DP? I have connected my headphones to "audio out" in back of U2410.