I would try updaint your sound card drivers see if that helps.
You can also try shutting down the laptop, removing the battery and then press and hold the power button for 5-10 seconds, than reattach the battery and plug ti back in to turn it on. It could be that there was some static electricity build up causing it.
If they doesnt solve it is the laptop under warranty you may need to contact Acer for a replacement (could be onboard audio chip which is on the motherboard is dieing).
Still I would try resetting it by removing the battery first. Static electricity can cause funny things to happen and is more common duing the dryer months