Speaking as the person who,
in ancient days of yore, played the
terrifyingly cute character littlefox (all lowercase!), I have to say that the accounts given above aren't quite correct.
The original foxish language definition was as follows:
The basic sounds are Yerf, Yip, Yarf, Yaff, Yiff, Growf, and Growlf. The general order of sounds from positive (happy, good, greeting) through neutral to negative (mad, unhappy, no), are: Yiff, Yip, Yerf (positive), Yaff, Yarf (neutral), Growf, and Growlf (negative). Murphle (a sound popularized by some other characters at the time) is a sound of contentment.
The basic sound, if modified by an -le suffix, is less absolute, less emphatic. ie: Yiffle is less emphatic than Yiff.
There are a couple of words that have somewhat general meaning, though the entire language is rather overloaded. Yerf's are generally used in greetings, though exuberant greetings are Yiffs. Growf means no, and a Yiff means a yes. (an example of the overloading of the language.)
There is one term that got defined by someone who shall remain nameless: Yipp is generally considered not polite, as it is basically a sexual proposition.
Later on, some
wolves redefined Yiff to more or less take the place of Yipp, as a generic term for sex. It soon became a noun, a verb, an adjective, and about as many other parts of speech as
smurf.
Oh, and yes, I'm aware that foxes have never made half the sounds listed above. They were cute sounding at the time (about ten years ago). That was reason enough.