I also have a dell, and tried many times to get sound, but have the same problem as OP. If connect the laptop direct to tv before turning the laptop on, then turning the laptop on i have on some very rare occasions had success. Normally i get lots of flickering screen (handshaking?) then a decent picture and no sound.
I have tried connecting through av receiver, and always get a decent picture but no sound.
Have tried variety of hdmi leads, and now given up. Assuming a fault with laptop, but I wouldn;t use it enough to motivate me to sort it out. However, if anyone ha an 'easy' fix.......................................