I assume you are inquiring about "liaison".
Please take a look at this page about "forbidden liaisons": http://french.about.com/library/pronunciation/bl-liaisons-f.htm
And also the pages about "required liaisons" and "optional liaisons"
No, it isn't. You would need an object in that case. "The boys are listening to" doesn't make sense. You'd have to say what it was they were listening to - "The boys are listening to the radio; .....to their teacher; ....to the rain;" etc. In this case, as there is no indication as to what they are listening to, they are just listening.