To suggest that the speed of sound varies intrinsically with attitude is nonsense. In fact, the greatest variation in the speed of sound (in a gas with a given composition) is temperature.
To quote Wikipedia: "In the Earth's atmosphere, the chief factor affecting the speed of sound is the temperature ... the effects of decreased density and decreased pressure of altitude cancel each other out, save for the residual effect of temperature."
To remove any doubt, questions about the speed of sound should specify a temperature: for example, "What is the speed of sound in air at 20 degrees centigrade?" The answer (according to Wikipedia) is 767 miles per hour, 1,234.8 kilometres per hour, 1,125 feet per second, or 667 knots.
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