Windows 7: Remote Desktop, local sound

So I have 2 laptops, my main rig (Windows 7 Home Prem x64) and an old XP Pro SP3 x86. The old XP is now a dedicated music player. I remote desktop without problem - it's even stopped asking me for my logon + password. These are joined via a 4port wifi router.

Both laptops also have minijack-to-phono leads from their headphone sockets to the inputs on the back of the same amplifier. So whenever I listen to music it comes out of the hifi - cool.

Problem - when I work downstairs and connect wirelessly from Windows 7 to XP via remote desktop, and open music, I can't hear it from the hifi - only the Windows 7 laptop's speakers (obviously being downstairs means I have unplugged my Windows 7 audio out from the stereo) but I wonder if there is a way to force the sound to go from the remote PC to the amp?

It seems strange that when I remote desktop, everything I do via RD is done on the remote machine (obviously!), including opening the media player, except music playback - instead of throwing the sound out of the headphone socket (and in turn the home stereo) it sends it over the wifi network and I hear it on the laptop speakers!?

Can anyone advise on this, please?