It may be because you have both the internal speakers and headphones enabled at the sametime on your laptop. This might just happen automatically when you plug your headphones in. You need to manage your audio devices on your laptop.
Go to Control Panel> Hardware and Sounds> Manage Audio Devices> Playback Tab (which should already be selected)
In the playback tab there should be a list of all the audio devices you are using on your computer. Right click on the one you don't want to use and click disable. This should stop the sound coming from that audio device.
Remember: You will need to enable that audio device if you want to listen to sound through it again. Your internal speakers won't work once you enable them again. This might also be the case with your headphones. You may also have to do this every time your plug in a device or once you restart your computer.
Another note: Sometimes disabled devices will not show up on the audio device list so once you disable a device it will dissappear. This is an easy fix too. Just right click on an audio device and make sure show disabled devices and show disconnected devices is checked.
I hope this helps.
(I was using a computer with Windows XP to explain the instructions but it should be similar with your Windows Operating System. You may however have to play around a little on your computer because it may be slightly different)