Is there any standard for the input impedance of an audio amplifier
There is a sortof standard for a certain class of professional audio gear.  This standard is 600 &Omega;.  All "line level" audio in such a setup is expected to drive 600 &Omega;.  This is also the impedance of the microphones.

However, many many amps don't follow this.  In fact, most amps try to be "high", which means to not appreciably load the output stage of whatever they think is producing the signal.  Of course everyone's idea of "high" is different.  The most common is probably around 10 k&Omega;.

That's what I've done with audio gear aimed at consumers.  Anything that produces a consumer audio signal won't be damaged, loaded to the point of distortion, or even appreciably attenuated by 10 k&Omega;.  Otherwise, you want the input impedance to be as low as possible avoid picking up stray noise when nothing is connected.