As many of you have probably heard, Hardy will be using PulseAudio by default to get a unified sound server going. MythTV does not yet work with PA; however, here's a configuration file you can change to make it. Open up your /etc/pulse/default.pa and on line 32 make the following change:
load-module module-alsa-sink device=plug:dmix
After logging out and back in, you should be able to get sound out of MythTV again.