Seems to be the same problem as is well described at http://freebsd.monkey.org/freebsd-ports-bugs/200708/msg01226.html
That is, when I am using "-r sound:local", sound seems to work, but after a while of usage the rdesktop session seems to hang after a new sound output finishes. If I launch rdesktop from a command line instead of selecting a GUI front end such as tsclient from a menu, when this happens I see an endless stream of error output to the console, saying "ERROR: select: Bad file descriptor". rdesktop consumes as much CPU as it can at such times, hence the (tight loop) in the summary above.
See the supplied URL for steps to reliably reproduce the problem, and if that doesn't work, I will look at it further to cook up a reliable set of steps.
That URL also contains a patch, which I didn't find in the CVS repository here, so I am assuming from that, and the fact that there are no bugs submitted here since 2004 with the word "sound" in the summary, that it hasn't been reported and acted upon here.