Sync licenses (eg to sync a song with video) require the songwriter's explicit consent in every case, but while a license fee is required to re-record a particular song, explicit consent is not required. That's why one-hit wonders' songs are often hard to find on places like iTunes or Amazon MP3 Store if you don't know the performing artist's name, because anyone can cover it, and there are entire operations who make a business out of producing terrible covers of popular songs to get some accidental purchases.

Any artists who haven't allowed their recordings to make it to electronic distribution are also drowned in poor covers online. AC/DC was a great example of this problem until they finally gave in and released their stuff digitally. There's still a ton of AC/DC cover recordings on iTunes.