If there is cause to believe that the sermons incited violence, there could be a case for subpoenaing sermons already given, but that is iffy unless they can give specific dates the sermons were given and the nature of the incitement.
If there have been no incidents of violence being attributed to congregation members who heard the sermons, it gets even iffier.
This would be similar to music or movies portraying and favoring violence....even if "fans" did commit violence after being exposed.
Were religion would come in is if, for example, a religion mandated that non believers be killed and a follower of that religion were to, say, kill a co-worker who was a non-believer in a manner practiced by that religion, beheading as an example, while claiming it was done in the name of the religion.
An alternative view is that it would be generic work place violence with not even hints of a religious motivation.