Have you tried to remove the driver? if not, right click on the device (in device manager) and click uninstall. if you are prompted to remove the driver, click yes. it will then ask you to restart. After the restart, it should have reinstalled the driver. if not, download it from Acer (if you haven't already.)
If that doesn't solve your problem, you may need to reinstall the CPU driver from the Acer support website. This is how I fixed the sound on my Acer laptop. Hope this helps! 
-PMiner