my sister has a dell inspiration, and the other day she had weird interference over the speakers and it is happening again now. It was a women's voice stating various numbers over what seems to be a tannoy system.
Your sister's laptop is picking up a radio broadcast called a "Numbers Station". Search for it on wikipedia.
A wireless antenna, by its very nature, is going to pick up unwanted signals as well as the desired wireless signal. Poor quality wireless receivers will have poor rejection of unwanted signals.
Live with it, or disable the onboard wireless receiver and use an external plug-in USB wireless dongle.