Although the application already contains the files needed to play videos, you might need to install additional Flash components like Adobe Flash Player. If it's configured correctly, the application will take advantage of the installed libraries and it will play those files. However, since it's a local solution, it means that the application will play only local files. If you try to open a URL, you will encounter errors.
Sound problem could also be related to missing codecs. Even if you have support for videos, it's possible the video uses another audio codec. Try to install Combined Community Codec Pack then play the file again.