From Rhymes With Orange, playing with morphophonology:
Some English nouns ending in voiceless fricatives (especially in /f/) voice these fricatives in the plural. There are three classes of cases:
(1) voicing obligatory in standard English: wife – wives, shelf – shelves;
(3) no voicing in standard English: fife – fifes, oaf – oafs.
Nouns in class (1) are subject to regularization; there’s some pressure to move them into class (2). Nouns in class (3) are subject to playful irregularization — yielding things like arves.