Intervocalic Voicing

Pronouncing F
Pronouncing V
Pronouncing S
Pronouncing Z
Pronouncing th
Pronouncing dh

What is it?

In Old English, the voiceless fricatives

became voiced when they occurred between two voiced sounds (usually vowels).

Definition

The historical sound change in which a voiceless consonant becomes voiced when it occurs between two vowels (or, more generally, between voiced sounds).

Old English Modern English Change
lufu love /f/ → /v/
giefan give /f/ → /v/
hæfde had /f/ → /v/ → later lost
wulf wolf No change (not between vowels)

Why does it not happen in Wolf?

Because it is not between vowels. But when it becomes plural, the f becomes v
F becoming V in plural conversion