how much energy is required to create a sound with constant frequency and amplitude?
That is impossible to answer with the information given. First, that 60dB is meaningless without a weighting and distance from the speaker. Even then, unless you're in a free field, acoustics and environmental factors come into play. Finally, it depends on the sensitivity of the speaker, efficiency of the amplifier, and rest of the electronic system. Figuring out how much power it takes is not a trivial or worthwhile task. In the real world, give it twice as much as you think it will take, and let physics work out the rest.