I don't know.. you're not selling it to me even though I'd probably actually not mind this. But this just sounds like something some sort of potion or "things" that could be created via herblore and/or invention maybe even prayer, not to mention it quite literally is pretty much doing harps at Prifddinas or at least that's how the "training" method comes across to me.
The instruments you list as well, I don't think a few of them existed during medieval times if you want to keep to that theme. One issue I can see with this is the animations of playing medieval instruments in the game, some instruments are almost impossible to play standing up; the hurdy gurdy, the uillean etc. comes to mind - medieval instruments are very bulky.
Another issue is Jagex would need to actually find and then purchase these instruments due to the fact they no longer record their tracks using MIDI, but in fact by recording the real instruments. Account for that and then the fact that very few players of such instruments exist as more modern instruments are a lot easier to learn/play and aren't as demanding in terms of maintaining them.
Creating them would fit the game nicely as crafting would be the perfect skill to use for this particular aspect but I can't see instruments being a standalone skill, maybe some sort of social gimmick where you can play an instrument anywhere in the game world - maybe.. instead of have it be for the single user make it a community centered "thing" (I'm reluctant to call this a skill) where it would benefit the user playing the instrument but also those around them. Your fishing example would be a good example of this.
Yet another issue which I just know will be a major issue - people will bitch and moan about hearing these instruments.
Hurdy Gurdy:
https://www.youtube.com/watch?v= Dord43GV4U4
Uillean:
https://www.youtube.com/watch?v=aF3fW4Nox9U
Bodhrán:
https://www.youtube.com/watch?v=n0NeLR9bPRc
Bones:
https://www.youtube.com/watch?v=Gb5bdEGwnS8
21-Dec-2018 15:16:54