Your purchase receipt is your best ally here, so keep it safe. Unfortunately, the store return policy doesn't apply to you anymore after 14 days, but that's the least of your worries. The important thing is that the player has a functional defect and that's covered under legal warranty, which in Spain covers two years from purchase. Media Markt is obligated to repair it or replace it at no cost during that period, regardless of the 3 months.
What nobody usually clears up is that you have two paths and one is faster than the other. You can go straight to Media Markt with your receipt and demand that they activate the legal warranty - they have to process it without asking questions if the problem is obvious. But if you'd rather skip the store, contact LG directly and they'll send you a warranty form; it generally takes longer because they have to verify the serial number and all that. My advice is to go to Media Markt first because you already have the relationship established and the receipt in hand.
One practical thing: bring the player in its original box if you have it, or at least well packaged. Stores sometimes make a fuss if you show up with the device stuffed in just any bag, even though they don't have a leg to stand on. And be clear at the store: don't ask for a return, ask for activation of legal warranty due to manufacturing defect. That word - warranty - opens more doors than return at this point.