I think there are two separate issues:
1) Code: The speaker wire is allowed to come out through low voltage boxes, etc. and is considered a low fire risk. Running it with power (tied together) increases the risk that you'll get a short between the power and the speaker wire. If there is a problem in the power lines, it'll (hopefully) be contained in the well designed junction boxes. If 120 gets shorted into the speaker wires, no guarantees. It shouldn't happen, but all it takes is one badly place picture hanging nail and you have a short. You'd have to check your local code to see whether it is a code violation, but it seems like a good thing to avoid.
2) signal quality: Running a 120 V 60 Hz line a long distance with unshielded, untwisted speaker wire will definitely give you some cross coupling. Your speaker wire will have a 60 Hz 'hum' to it. Whether that matters at all is completely dependent on the magnitude of the hum relative to the normal signal. If it's low, no big deal. If it's a big hum, and it's to a full range speaker, you could hear it.
General leaning: it's usually easier to avoid running power with speakers than it is to prove that it won't be a (very hard to solve) problem.