How does the earphone cable in a mobile phone pick up the FM reception?
Your question is based on a misunderstanding. The RF signal is not directly sampled by the sound card. 

Instead, the earphone cable just doubles as an antenna for a dedicated FM receiver; the soundcard hasn't got anything to do with FM reception.
In fact, it doesn't even "get" radio frequencies – they're filtered out before and fed to the FM receive chain.