Combining Yellow and Blue light does not give green light. It makes white light. Light is additive color mixing. 'Yellow and blue make green' only refers to paint. If i have paint that reflects yellow, in actuality it ABSORBS all other colors besides yellow (subtractive color mixing). So by mixing paints I DECREASE the spectrum of light that is reflected, and get green.

Adding waves isn't as simple as averaging the frequency. Mixing two waves with different frequencies actually gives a wave with two parts: one part that is the average frequency, like you said, and another part that is the difference between the frequencies, which creates "beating."
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So yes, the result is a wave that has a frequency similar to a 'B' when you hit those two notes at the same time, but it sounds different because of beating. Real music is also comprised of a bunch of different harmonics and beats, it's not just pure sine waves for each note. So you get a much more complex sound by hitting chords.