Correct, (annoyingly) there is no volume control (or mute), pressing triangle will bring up the main menu, you will need to go to Settings and Audio to set the PlayTV Volume Level. This is NOT a proper substitute for volume control and is really just to set the output level similar to that of other devices connected to an amplifier. The Volume Level settings go Min -5 -4 -3 -2 -1 0 1 2 3 4 5 Max.
Sony are releasing a new controller (in Japan at least) with additional IR controls for your TV and AMP, and that includes volume and mute buttons, along with PlayTV labels (so no more need for sticking on overlays). Google the model number CECH-ZRC1J for full details. Needless to say, I wants it, my precious :-) This is the best picture I've found of it so far - http://ns3.netartz.com/~fammys/imgdb/item_s/90/s209007290655.jpg