You make the "poster" sound unreasonable!
Surely, "silent mode" should mean what it says. If HTC want to leave the thing as it is they should call it something different ... I don't know what, maybe something like "Ringer Off".
"Power off" is an obvious option (and I used it myself in a concert at the weekend), but it would be nice to have the option to go quickly into a truly silent mode and still have vibration notifications.
Thanks for the Apps suggestions, Joe Jack and Jim. I like the idea of separately controlling alarm and music volume. This might help to prevent deafness caused by listening to music through headphones when an alarm goes off!