cannot hear sound via HDMI

My laptop connects to tv through hdmi cable ok(sound and pictue by tv). However when my son connects his Acer laptop(win7), using the same cable and tv, we can see the same picture but no sound via tv. We can hear the sound through the laptop speaker only. We tried to change the property, sound mix etc without success.

Is that using the standard Windows audio manager or the one bundled with the graphics chip? With the latter you should find an option to tell it to select the HDMI output.

Update to the audio maybe required.

It took me 3 attempts to get my Dell Studio (win7) to work correctly. Unticked and ticked various options within Control Panel - Sound.