The correct answer is option 2. It states that the loudness of sound is related to its frequency, which is incorrect. Loudness is actually related to the amplitude or intensity of sound. The frequency of a sound wave refers to the number of vibrations or cycles the wave completes in one second, and it determines the pitch of the sound. So, option 1 is incorrect too.

Option 3 is correct. A musical sound is made up of a fundamental frequency and harmonics, which are integer multiples of the fundamental frequency. These well-defined frequencies give the sound its musical quality.

Option 4 is also correct. The timbre of a musical sound is related to the waveform of the sound wave. It refers to the quality of the sound that allows us to distinguish between different instruments or voices producing the same note. The waveform of a sound wave represents the shape of the wave and influences its timbre.

In summary, option 2 is not correct as loudness is related to amplitude, not frequency. The other options provide accurate statements about pitch, harmonic frequencies, and timbre.