bee-bo/bee-bee/bee-buh is a statment proclaiming extreme cuteness, it is pronounced the way it is spelled but you must say it very high-pitched while wearing a grin of utter satisfaction to get the meaning across...
Eric: Hey Elana, whats going on?
Elana: Not much, you?
(a kitten or other cute little animal walks past)
Elana: Ohhhhh, bee-bee!!!