If you are judging relevancy based on SEO factors, I would say you need to some form of keyword targeting on your articles (e.g. "headphones" appearing in your article). By this, I don't mean to spam keywords in your articles, but to include them sporadically and seamlessly so that your article can rank for that certain keyword.
But from a marketing point of view, I feel it would be great to occasionally have articles that are not specifically related to headphones, but in the same niche area (e.g. music). For example, writing about the Jay-Z album in a headphones related site is perfectly fine, since you can assume most headphone lovers are also music lovers. Writing about trending topics in relation to your site's niche area is possible too, and would certainly help to offer a fresh perspective in your site.
I'd imagine it would be quite boring if all the articles in your site are about headphones. Going slightly off the main topic would give a personal touch to your site, and for all you know, it might actually be popular and attract a new group of audience. Go for it and let us know how it goes!