I agree that a question on caring for wood, for example, would be very well suited to the Arts and Crafts Stack Exchange, however the community here will likely know much more about what guitars are made of / coated with etc., so it has benefit here. Additionally, someone wanting to find that out in future is more likely to look at a music site. If it doesn't get suitable answers here I have no problem with migrating it.
Similarly, questions on maintenance of electronic instruments are on topic here. Many would also be suitable on Electrical Engineering Stack Exchange though.
The music through headphones one is definitely on topic here though (as well as potentially being on topic on Sound Design Stack Exchange) as for many musicians it is a key element of practice.
From the help page:
Music: Practice & Theory Stack Exchange is for musicians, students, and enthusiasts. If you have a question about...
- practice or performance technique
- music theory, notation, history, or composition
- technical analysis of a specific, complete work, or well defined section thereof
- instrument maintenance or repair
- usage of specific music software or hardware
If they do annoy you, you can block specific tags from ever appearing in your feed (I do this on Arqade, as there are so many questions there on computer games I have no interest in)