External vs Internal Speakers - Keeps switching to internal?

Hello,

I recently purchased a Studio 1558 laptop. The specs are below:

Processor: Intel (R) Core (TM) i7 CPU   Q 720 @ 1.60 GHZ
RAM: 6 GB
System type: 64-bit operating system
ATI Mobility Radeon HD 5470
Sound - ATI High Definition Audio Device, IDT High Definition Audio CODEC

I have plugged in older Dell speakers that I used with my desktop. The sound works fine until I load a webpage, or load a folder etc and then sound will temporarily switch from my external speakers to my internal laptop speakers. This is quite annoying as it happens frequently and sometimes I blare my externals which can't be good if it keeps switching to my internals.

Can anyone recommend a fix? It would be greatly appreciated.

Ryan