I was able to fix my problem by calling the saySilent-method after a longer period of time. It would seem that the SitePal avatar needs to be fully loaded before one can call on the API functions.

I saw the examples but I am still unable to use functions such as saySilent. I noticed that saySilent is called from vh_player which is defined as:
vh_player = MovieClip(ldr.content);

However when I also try to call the method from vh_player I keep getting an error saying:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at VHSSPlayerV5/saySilent()
at Avatarwebsite6_fla::civa_5/completeHandler()