Re: drywall over plaster and speaker wire.
Can you overlay plaster with drywall? Yes, this is done all the time. You can also tape and float the cracked plaster with drywall tape and compound, saving the expense/step/work of hanging drywall.
Can you surface run the speaker wire? Yes, but it's not recommended. What happens when you hang a picture, plasma tv, shelving, etc., and inadvertently hit the wire with a tool or fastener?
A better idea would be to either use a surface mounted wire mold or to notch the plaster and run the wire inside the wall. Even better would be to remove the plaster entirely, run the speaker wires and any other wires and plumbing while the wall is open, then install drywall. This will allow you to insulate if necessary and keep all the important things protected within the wall. You will likely have to apply shims to the studs to flush the drywall with the existing outlets, door, and window framing.
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