A science fair topic that would attract immediate attention is one that demonstrates an audience's reaction to volume. Consider posting index cards with the hypothesis about people's reaction to both very loud and very soft music but cover them with black construction paper so it can be lifted up and read. Randomly, play both "dynamic levels" with no explanation as to why. After the onlookers react, have them flip up the construction paper to read if the hypothesis was correct.
Another manipulative variable for a science fair topic would be the adjustment of frequency and acoustics. Sound is powerful. The right frequency can shatter solid matter. For a science fair, playing with the frequency of sounds can be done with a music editing program. While all sounds emit a frequency, computers can adjust that frequency to cause certain reactions in the listeners. For example, if frequency is the manipulative variable, then by adjusting the frequency of a sound and leaving the volume level as a constant, there should be certain reactions seen by the people who are able to hear it.
The manipulative variable of rhythm and beat is at play whenever someone listens to the radio. Most times, people do not realize they are tapping along to a beat due to the initial visceral reaction of the body reacting before the conscious mind. Randomly play pieces of music with beats and pieces of music with ambient rhythms. With this as the manipulative variable, the science project could also predict the reactions of people who are within earshot.
Choice and preference are other manipulative variables you can use in a science fair project. Not everyone can listen to any type of music, and most people have strongly negative reactions to a specific type of music. The science project could focus on the two main polarizing genres of country and rap, but by delving deeper into specific types of rock, different periods of classical concert music and different regions of world music, there can be a visual (or statistically data-driven) result of how may people enjoy the different genres.