Unusual behaviour of sound drivers

Hello,

I have a compaq 510 laptop with Windows 7. I usually use a headphone to listen to music or watch movies. Now at the time when gtalk pings or i try to do the soundcheck of my headphone at the contro panel, the headphone gets disconnected and the sounds starts orginating from the laptop speakers, try however i cannot get the sound back from the headphone. What should I do the get the headphones activated again?

Regards,
Anirban