I'm guessing you get sound without headphones. Is this correct?
Do you have a bluetooth speaker, or headset you could try(or a friends)?
I'm unsure of what the issue with your sound is, BUT I think knowing those two things would help figure it out.
I would say if you get sound from the internal speakers, and from bluetooth, it could be a hardware issue with the headphone jack, or something similar. If you dont get sound in one or more of the other instances then it could indicate a software issue or another hardware component.
If the device is under waranty and it seems to be a hardware issue I would consider getting Acer to fix it. In which case you may want to downgrade back to stock before you ship it.
Hope this sound issue was just a temporary thing that is already resolved. Let us know what happens