The release of SQL 2012 default to dynamic ports for remote access. You need to open port 1434 for the SQL Browser if you want clients to access it remotely.
Original Post Here by Jeffrey Palermo
This TechNet article explains that to connect, the client will send a UDP packet to port 1434 first to resolve the dynamic port. Then it will connect as normal.
The appropriate Windows Firewall rule to establish is a program rule, not a port rule.