Most powered speakers only draw a few amps. A 16-AWG cord is plenty for a 15 foot run. You could even get by with thinner 18 AWG, if you can find it. The thinner the gauge (higher numbers are thinner) the more flexible the cord given the same insulation and outer jacket type. The key thing to look for is really the outer jacket. These have all sorts of ratings (S, SJ, O, OO, W, T, E, etc). You may see SJT or SOOW or things like that. The E rating is a more rubbery compound that tends to remain more flexible in colder weather. For instance a 16AWG cord rated SJEOW should work well in the cold.