I challenged the fine, but my appeal was turned down and the fine increased and increased over time. In the end I had to pay it but it was worth it to protect my ears.You were smart for wearing earmuffs. In my case, airbags turned moderate loudness hyperacusis into the worst case of noxacusis ever. I'm surprised you got fined but that is better than making your ears worse.
If I did not have any money or property, in the end the Spanish government would not be able to charge me that unfair fine, but in Spain if one does not pay an administrative sanction, a fine, a parking ticket or whatever, and one has work income or property, then either the fine is deducted directly from wages or property is encumbered and could ultimately be auctioned to pay the fines, sanctions or taxes due...
Actually in Spain sometimes the person who commits an "infraction" does not even know, because the notification procedure can fail, but there is a way for the government to recover the money without the infractor being notified. And after a certain time elapses, they just take your money, usually from wages.