Lord Tarin wrote:
Modern-day trumpets are advanced, but I was thinking more along the lines of the quintessential trumpet that one imagines when thinking of the middle-ages: long, slender, no valves.
Well ... I don't tend to think of "trumpets" in the Middle Ages at all. (Partly, when I have my worldbuilding hat on, I hardly ever think of our world's medieval period; my world's initial culture is fairly medieval, but includes bits from all over.)
Lord Tarin wrote:
Maybe they were referred to as horns then, but I have horns (think Boromir) as well as trumpets just to avoid confusion.
Fair enough ... but when talking about our world's history, I tend to think of various 
kinds of horns: ram's horn (i.e. shofar) in ancient Hebrew worship, Alpenhorn, etc.
Lord Tarin wrote:
Dulcimers and bowed instruments? Those I hadn't thought of.
Dulcimers are something I would think of because my dad plays the hammered dulcimer and my mom the lap dulcimer. And bowed instruments came to mind because you mentioned violins as too advanced ... but it seems to me that they're complicated mainly by the amount of detail required to fit the modern specification, rather than by the notion of bowed strings over a soundbox. The bowed psaltery, while actually invented in the 19th or 20th century (I think), could have been invented anytime after the principles behind the viol family were discovered.
Lord Tarin wrote:
Really, I haven't focused too much on the musical aspects of my work. Just another indication that I need to delve deeper into worldbuilding before I actually begin writing!
Not necessarily ... will this ever come up? It's entirely possible to spend too much time delving into corners of the world that have nothing to do with the story, and never actually write the story. This is just an interesting cultural detail (and you'll note that my post about music in my world focused more on the culture than the specific instruments).