Still TDA2050 single supply circuit
If amplitude large than 7.5v, the waveform will change. For acoustic signal, will its pitch change?


The pitch doesn't change when a signal is clipped, you only get increased distortion.


  If this as a real amplifier, as volume increase, the sound sounds strange. How to solve it?


I assume you refer to the effect of increased distortion, lower the output power or increase the supply voltage to lower the distortion.