[Salman]

Salman asked the Naked Scientists:
   
Where do sound waves finish? And are there waves that have finished or exist that we can't hear?

What do you think?

[RD]

Where do sound waves finish?

In a dead heat  [] ... http://en.wikipedia.org/wiki/Heat_death_of_the_universe

are there [sound] waves ... that we can't hear?

Yes infrasound, ultrasound, and the sound of Jehovah’s witnesses knocking on the front door.

[JMLCarter]

Sound waves attenuate rapidly beyond detection by the human ear. The energy in the sound wave ends up as thermal energy in the medium through which it passes (the air is warmed up).