When you had video and audio from you 24" LCD did you have ANY other audio cables (like an 1/8" analog stereo cable) attached to your monitor as well? To verify that you were getting your video and audio through your HDMI cable to your 24" LCD monitor make sure that NOTHING except the HDMI cable and a power cable are plugged into the monitor.
If after this if you are 100% sure you are getting audio through the HDMI cable then I would check the audio settings of your 55" LCD. Many of TVs that take an HDMI cable have settings to accept digital video over HDMI while using analog audio from a composite source. Make sure everything on your 55" LCD is set to digital for your HDMI connection.
Also make sure you are not using any adapters (DVI to HDMI). Sound cannot be passed through a DVI adapter (plus it will break the HDCP compliancy).
when you connect it to the TV does the default audio device change? – Xantec – 2010-12-03T04:08:40.350