Sound Device Output getting changed automatically

Hi,

This has been bothering me for a while now. I have a HP Compaq 510 Laptop running Windows 7 Home 32bit. My speakers are Altec Lansing.

Here's what's happening, whenever I'm playing music from my winamp then suddenly I get a call from Yahoo Messenger or if I use Skype then the sound output suddenly switches to my laptop speakers instead of the Lansing. This happens when I'm using other softwares too like Mixcraft. I had to restart my computer for my Lansing to be detected again.

Can you help me with this problem?

Thanks