YouTube videos have embed codes below each video.  Recently, they switch the coding to have iframe coding which will not display properly when used in a placemark on Google Maps.  There is a solution to this little problem–you will need to select the box for “use old embed code” provided below each YouTube video.  This will provide you with an embed code that can be used in Google Maps placemarks.  Here are some annotated screen shots to help walk you through the process for using YouTube videos in Google Maps.

Finding the embed code for the placemark:

Embedding the code into a placemark: