First off - have you powered down fully, unplugged the magsafe from the wall for 30 seconds and then restarted?
When you go back into the sound Pref's after selecting the TV:
A. It has reverted back to the internal speakers being selected.
B. The TV is still selected.
If the answer is Yes to:
A. Try moving the sound pref file to the trash and then attempt it again. It is in:
Users / your user / Library / Preferences / com.apple.soundpref.plist
B. Try a
SMC reset.
You'll need to go back into Sound Pref's and select the TV again after each one of these. If the TV is already selected after one of them, change the selection to the internal speakers - close Sound Pref's - open Sound Pref's - change back to the TV.
After you have tried the appropriate one above, it won't hurt anything to try the other one. If this doesn't fix it, I'd be giving Apple a call at that point.
(Well, I might would try creating another user account to see if it happens in that user account also before I called Apple. This would help determine if it is a system setting or something related only to your user account.)
Which mdp to HDMI adapter are you using?