Acer Chromebook sound problem
I've been having periodic problems with sound. Mostly, it's triggered by switching I do using an HDMI KVM I use. The problem was that sometimes after switching to the Chromebook, sound was routed through the HDMI cable rather than out the headphone jack or sometimes it seemed like the volume was off altogether.
One solution was to unplug/plug-in HDMI, then sound would route through the headphone jack. In other cases, it had gone into mute, so toggling the mute button fixed the problem.
I hope this helps if you have similar problems.