Dell Inspiron - bad echo on sound

I have just bought a brilliant Dell Inspiron M5030 laptop.  However the sound is very echoey, and I can't seem to get any advice as to how to fix it, save an attempt on a Microsoft site that gave me wrong instructions.  Can you tell me how to fix this?  The serial number for my machine is:  < ADMIN NOTE : Service tag removed per privacy policy >

I am re-posting this as no-one has been able to answer me yet, and it is causing a lot of disruption as I am trying to follow an online course with videos I can hardly make out, because of the echo.  It sounds as though the guy is in a huge empty warehouse.

Can someone at least point me in the right direction?