Best answer by Ryan E - Sonos
The Start and Stop Time options you see in iTunes are an iTunes-specific function. Unfortunately, these options will not carry over to Sonos since the information is not embedded within the file itself. When Sonos plays the file in question, it will play it from start to finish.
However, you may want to try enabling the Crossfade function on Sonos when playing the playlist. This will not have the exact effect you're looking for, but will provide a fade transitions between tracks which may cut down on the time between the end of one track and the beginning of another. You can enable Crossfade on your various controllers by following the steps in this FAQ.
Let me know if you have any additional questions and I'll be happy to help.