ROB: First of all, Ben is right. Second of all, the Issur of listening to music during Sefirah is mentioned as far back as the Pri Megadim, which, historically, is when music became accessible and popular with the average person. Thus, we already see that when people started listening to music, the Poskim began to apply the Ikkar Hadin of all year round to, at the very least, Sefirah.
Also, your entire perspective on this is wrong. All the Nihugei Aveilus of Sefirah are Minhagim, not Dinim. The Minhag just about everywhere became to avoid music during Sefirah (so some are Noheg to only listen to slow music and some, but very few, will listen to recorded). Just because one Minhag is older does not change the fact that music became one of the main Nihugei Aveilus of Sefirah. It doesn’t matter how recently this Minhag started. It barely matters why. The fact is that the Minhag is to not do it and you can’t change that.