Skin effect is measurable at audio frequencies but at those frequencies the small difference in surface area between solid and stranded is an insignificant factor. Non-listening tests have born out the fact that a wire with a given cross section of copper and the same length, whether solid or any configuration of stranding or braiding, will conduct audio frequencies measurably the same.
Another benefit of stranded wire relates to vibration but maybe I am wrong there too?
Stranded is certainly more flexible and will tolerate bending and vibration much better than sold, yes. How that would relate to a speaker installation I don't know unless you are talking about a portable setup or where speakers will be moved about. Nothing wrong with using stranded wire for those reasons or just because you want to use it. But don't think one or the other is going to make your speakers sound any different.
-Hal
