Default Sound from internal and external speakers

I plugged some logitech speakers into my A500 32gb model tablet and the sound is coming out of both the tablet speakers and the logitech speakers at the same time :S its extremely annoying as I dont want to hear the tinny crap coming from the tablet. I couldnt find any option to turn that sh*t off and the problem does not persist with headphones :S

Am I missing something here?
Does this happen to you??