There's a setting in there (I think it's under audio settings) for "Multi-output" if you turn that on the sound will travel through both though after setting the display it should as Ni72ous says give you the choice of where the audio is to be transmitted from.
I'd be careful to make sure you turn off things like DTS etc if your amp doesn't support them otherwise you'll get some weird audio problems that are hard to pin point.
You don't really need to have the Audio Multi-Output option on - just be sure to select the sound output to be put through AV Multi.
Also - while Multi-Output is on most of the higher Dolby etc. sound formats will be downmixed so use this option only if you really need to have both outputs active at the same time (or if you're to lazy to change it everytime you need to re-direct sound from HDMI to AV and vice versa).