Perhaps a month ago or earlier, YouTube started to change the embed codes they provided below each video. The switch was to an iframe code that would not display properly when embedded into a placemark on Google Maps. The solution to this little problem is to select the box for “use old embed code” provided below each YouTube video. Selecting that box will provide you with an embed code that can be used in Google Maps placemarks. Then just copy that code and proceed as you always have to embed videos into Google Maps placemarks. I’ve included some annotated screenshots below. If you would like directions for the whole process of using multimedia in Google Maps, please see the Google Maps section of Google for Teachers.
Finding the embed code.
Embedding the code into a placemark.