Check that the volume button on the side isn't at minimum. Go into APPS > Settings > Sounds to check that the sound hasn't been muted there.
If that doesn't help . . .
A similar problem on a different Acer model is mentioned here:
http://www.androidtablets.net/threads/acer-iconia-tab-a500-no-sound.45112/
One post there reads: "I installed Sound Manager, enabled the mute profile, then enabled the loud profile, and hey presto, sound again!"
So that might offer a possibility:
https://play.google.com/store/apps/details?id=com.cgg.soundmanager&hl=en_GB
If you still don't get anywhere, phone Acer Customer Care on 0371 760 1020 (Mon - Fri 9am -5:30pm).
[03 numbers are charged at the same rate as normal 01 or 02 numbers]