What is the earliest source for the prohibition to listen to music during the 3 weeks?
Is there any dispute as to the timeframe of the prohibition (only 9 days, week of etc.)?
I found this:
The sources seem to be Mishna Brura: תקנא/טז and Magen Avrohom תקנא\י
Kitzur Shulhan Aruch says that it is permissible for someone whose profession is musical instruments to play in a Goy's house, that implies I guess that it's prohibited otherwise.
You're right, it's a difficult question. Everyone knows it's forbidden, but it's difficult to find sources.