We know, for $q=2$, there are 4 quantization levels: $\{-0.75, -0.25, 0.25, 0.75\}$. However, looking at the waveform of the quantized signal we see, that the levels $\{-0.75, 0.75\}$ are rarely used and mostly the quantized signal jumps between $\pm 0.25$. Looking at the original waveform, this behaviour is clear: Also, the original waveform mostly shows smaller amplitudes. Accordingly, the quantizer switches between the two levels $\pm0.25$ most of the time, wasting the potential for signal improvement of the remaining 2 levels. We can also hear this effect in the quantization noise: For louder signal portions (e.g. around $T=8s$), the noise becomes more uniform than in the quiet parts (e.g. the beginning of the audio).

What can we do to improve our quantized signal? Clearly, we can increase $q$. But, as a tradeoff, we can also reduce the dymanic range of the quantizer and hence let clipping occur. At the positive side, we get the $4$ levels of the quantizer used more uniformly, and hence reach a better SNR in overall. See, what happens, if we set the maximum quantization amplitude to $U=0.5$: