Simon, here what I could find on a HP site. It worked with me:
http://h20015.www2.hp.com/hub_search/document.jhtml;jsessionid=
MDSK2PGRCF53BQFIOGBENZQ?reg=na&cc=us&docName=lpg40191&lc=enInformation
To listen to music with headphones on the DVD/CD drive, digital audio must be disabled for the drives in Device Manager or Multimedia Properties. Disabling the digital audio option will not have an effect on other devices on the system.
In Windows XP, digital audio must also be disabled in Windows Media Player for the CD recorder in order to hear audio with headphones.
Windows XP
1. Click Start, Control Panel.
2. Double-click the System icon.
3. Click the Hardware tab, then Device Manager.
4. Double-click DVD/CD ROM drives.
5. Double-click the HP CD-Writer drive and click the Properties tab
6. In the box labeled Digital CD Playback, remove the check from the Enable digital CD audio for this CD-ROM device box (see Figure 1).
7. Click OK and restart the system if prompted.
8. Insert an audio CD and try listening to the music.
Disable digital audio in Windows Media Player (Windows XP only)
Use the following information to disable digital audio in Windows XP.
1. Click Start, choose All Programs, choose Accessories, choose Entertainment then choose Windows Media Player.
2. Click Tools on the Windows Media Player menu bar then choose Options.
3. In the Options window, click the Devices tab.
4. Highlight DVD/CD-RW Drive, click Properties (See Figure 3).
5. Click the Audio tab in the Playback section, verify that Analog is selected (See Figure 4).
6. Click OK, and then OK again to save the settings and close the Media Player Options window.
7. Insert an audio CD and try listening to the music.
Hope that helps.
* Simon edited url link to fit page.