I think you are talking about "The Rains of Castamere". That song started playing before everything happened, and that song is famous because it is associated with the Lannisters, and thus Catelyn had reasons to figure out what was going on.
Yes, keep reading, the second half is indeed among the best in the series.
I'm pretty sure that this will be the ending of Seasons 3, since they are to split Storm of Swords into two different seasons.
--
Dr Football > Dr Soccer