If the wave speed is constant and the wavelength changes, it means that the frequency of the sound wave emitted by the loudspeaker has changed. The frequency of a wave is inversely proportional to its wavelength, so as the wavelength increases, the frequency decreases.
In this case, the loudspeaker started vibrating at a frequency that corresponds to a wavelength of 1.0 m, and then it changed its vibration to produce sound waves with a longer wavelength of 1.5 m. Since the wavelength increased, the frequency of the vibrations decreased.
To summarize, the vibration of the loudspeaker changed by decreasing its frequency as the wavelength of the sound waves it produced increased.