Ventrilo/voip problem

So I got a new computer recently, and with the computer I am currently using HDMI tv screen thing as a monitor. Unfortunately, with this when I plug my headset into my computer, the headset (or pc speakers btw) dont work and I cant use voip...

can anyone help me here?

What version of windows are you using ?
Wife uses win 7 and we had to check the setting for sound as theres a setting that each program can have its own volume control.Then we had to use control panel and set the pc to use its front audio ports so headphones worked.
i swear with all the gd stuff in win7 - i still prefer certain parts of xp and the same parts in win7 seem harder to setup ( never had any audio probs with xp and was like the saying "plug and play" )

Is it not a case of setting Ventrilo to use the onboard sound, rather than the HDMI on the graphics card?

Check to see what sort of audio control panel you have. Check the various settings in there to see what audio outputs you are using, and what jacks are assigned to what function.
It sounds like you must have the audio set to output to HDMI. You'd want to check to see if you can also output to speakers.
On many audio systems you can reassign the various audio jacks for different functions, so make sure the jacks you plug the headphones and/or mic into are actually set for audio (front) out and mic input.

In Ventrilo, check what the settings are for your input and output streams. If you want to keep using the HDMI, you may have to select the hardware instead of Direct Sound, but if you are only using this HDTV as a monitor, and you will be using the computer's speakers, you could just disable the HDMI output and use the normal speakers and Direct Sound.