Sorry if this topic has been raised before, but I wanted to include in my map a long dialogue by Wheatly. But I only want a part of it to play, not the entire sound file. Is it possible to achieve this?

You can. If you only want to play the start of a sound, you can easily play a sound (through an ambeint_generic) with the PlaySound input and then wait a certain amount of time before using the StopSound input to stop the sound. - I found that in portal 2 StoupSound is a bit stubborn, so try setting the volume to 0.2 (or 0 if you are not to play it again).

If you want to play a sound mid-way through the sound, you can try fiddling around with the Volume input and then set the volume back to full at the time you want to hear the sound. So None = 0.2 (lowest I found without it resetting the sound again), Full = 10. Make sure to have a delay (like 0.1) on setting the volume to 0.2 though, otherwise it will not work.

This may take a bit of trial and error if you want to get exact timing.