Reason why polarized electrolyic on audio line doesn't blow up
Although it is desirable to bias electrolytic capacitors with a DC voltage they will work without problem with zero bias.  The aluminum oxide layer that acts as the dielectric is created during manufacture and is fairly robust and long lasting.

They will even survive a short amount of low level reverse bias although it is not recommended.

Electrolytic capacitors can blow up if reverse voltage is applied or excessive bias in the correct polarity but in general that will only happen if there is enough current to cause significant power dissipation such that the device heats up significantly.