If I connect the HDMI to my television and the headphone socket to my video amplifier (I only use the TV as a video monitor), this messes up the iPad, by stopping the sound in some programs, in the internal loudspeaker and headphone socket modes.
If I play tracks from "music" with HDMI and external amplifier, it works, but this is the only program where it does. HDMI works on all programs but not the sound. I have a iPod 2 iOS 5.

I believe this is an app issue - i.e. it's up to the individual Developer as to how they support audio and HDMI. We have a couple of Members who are professional iPad app developers, so hopefully they will come along soon with a definitive answer.

Tim