I could not find a lot of information on this other than some stale pages (like feature request 18 on Source Forge.)
It looks like Mumble does not currently look at SRV records prior to just using the A record in the current 1.2.12 branch, but there is some discussion into using QT 5.X's implementation of it. Looking at the 1.3 snapshots, QT 5 seems to be used though, at least for the Windows version.
Will 1.3 have SRV record support in an upcoming build by chance?