Orientation of an electrolytic capacitor in a circuit
There could be many reasons why polarity is not indicated.
When thinking about this, you must remember that the capacitor must pass audio which is an AC signal, and it must block any DC bias if present.

Not indicated because it defines you have to use a non-polar capacitor. Bipolar electrolytics have existed and have been used for audio. The LM386 is extremely old device and 10uF value was reasonable size for bipolar electrolytic technology back then. These days 10uF ceramics exist, but whether or not they should be used for audio due to non-linearities is another thing. A bipolar electrolytic is basically nothing more than two polarized electrolytics in series so you can build one from two 22uF capacitors.

Not indicated because it depends on the circuit. The input audio could have DC bias from previous stage, and LM386 input has approximately 0V DC bias, so polarity would depend on if the previous stage had positive or negative bias compared to 0V.

Not indicated because it can't be defined. If you have an audio signal with 0V bias, the capacitor can be put either way, as the bias will be 0V at LM386 input too. It will actually not be exactly 0V but near 10mV so close enough 0V. Since there is virtually no DC bias the capacitor could be removed from the system as it is not necessary.

Not indicated because it does not matter. Assuming the amplifed audio bias is almost 0V and the LM386 input can take amplitude of +/- 0.4V audio signal as input, the capacitor won't have voltage above 0.4V over it in any direction even for long steps, so a standard polarized  electrolytic capacitor will function in the circuit just fine.