lightning storm

You should have immediately unplugged your audio system from the wall outlet.

It's possible slight damage may have occurred to some circuit board traces. It's called electronic rust. 

I would suggest you unplugged your audio system from the wall outlet during lightning storms. 

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Yes, it is probably EMI either entering directly into your system or EMI affecting your power lines.  In any even, it is better to turn everything off during a storm.  Even better is to unplug the gear because a direct hit of lightning could be powerful enough to jump the switch in your gear.

Even when lightning does not directly strike a power line the magnetic pulse is picked up by every cable nearby, from high tension lines to your house wiring, antenna cables and Internet provider's wiring.  

We don’t always know when a surge will come down the line so having a good surge protector is essential IMHO, but even then I would absolutely recommend unplugging when you know a big storm is coming through.  Sometimes I even turn off my HVAC and range as well, despite having a whole house surge protector.  

Thank you for your responses. Much appreciated and I will be turning everything off and disconnecting in the future.

How is "electronic rust" detected and repaired?