El Corte Inglés does accept returns without the original box, but there's an important nuance worth clarifying. As long as the product is in perfect condition and within the 30-day window (which is your case), there shouldn't be a problem. What matters more is that the earbuds work correctly, that they don't have dents or signs of excessive use, and that you have your proof of purchase or can identify the transaction.
When you go to the store, bring the earbuds clean and in the best condition possible. If you still have the receipt or access to your account if you bought them online, that makes things a lot easier. El Corte Inglés tends to be pretty flexible with this because they know people don't always keep boxes, but here's the thing: if the earbuds look damaged or like you've been using them for weeks, then they could reject the return arguing they're no longer in sellable condition.
Best thing to do is talk directly to customer service before you go, even if it's by phone or through their website chat. That way you'll confirm specifically what the policy is for your purchase and won't waste time going for nothing.