5510: Audio from websites not working when external monitor is also hooked up. (But Skype sound still working)

I couldn't find the answer to this. Apologies if it is already posted somewhere

I have a Dell Precision 5510.

I have it hooked up to an external monitor using dual screen

The sound from a video on youtube (etc) works fine when the external monitor is unplugged, BUT NOT WHEN THE MONITOR IS PLUGGED IN. This confuses me.  Is there a solution?

Thank you.

cecilia