Gibberish Sound Effect
There is no one gibberish sound effect for LEGO minifigures.

Traveler's Tales (the developer of many popular LEGO video games) creates its cutscenes with a mime/sound effect style similar to movies found on the LEGO website.  Rather than record character voices with words (as Warner Brothers did with The LEGO Movie) Traveler's Tales recorded voice actors to mimic gibberish murmuring, grunting, and other non-lexical sounds.

If you would like to emulate this filmmaking style, I suggest you use a microphone of your own and record the sounds yourself.  Have fun!