My son last night had a science question on his homework that asked how a change in frequency affects the speed of a wave. I could remember the answer, but not a good way to explain it. I thought there must be a decent Youtube video out there showing what happens, but on stuff like this, it can be surprisingly hard to find a good video for a high school level.
The best I could come up with, and it is pretty good, is from the Khan Academy talking about the speed of sound. (It's the second in a series, but I don't think its essential to watch the first. It also has more detail than necessary, but in the last 60 seconds, they deal with the frequency/speed issue pretty well.)
So, for all other parents with a similar problem, here it is:
Speed of Sound: