I was too lazy to read through everyone's essays, so I'll just state my beliefs.
Sheet music:
Dependent on whether the composer is alive or dead. My 'pirating' of a chopin piece that is public domain (given that the sheet is still pirated from a publisher) is hardly unethical. If a composer is alive or recently dead (in that their family is still dependent on royalties), I'd never pirate it.
Youtube:
I saw mentions of Lisitsa, so I'll speak to those comments. If she posts performances on youtube, that is a conscious choice to put her performances in a publicly accessible manner. If someone else posts it, it's unethical to rip the music off for your own personal use. If I post a performance of myself on youtube, it's perfectly acceptable to rip it off. It's not about their career intentions but about conscious choice how their art is released.
Generalized musical performance:
Given that these performers usually aren't performing pieces they wrote is irrelevant, since their potential to sell the music is about artistic interpretation, not about composition. The conscious choice to sell the sheet music by composers means they are allowing others to perform it how they choose, and the restrictions on the music are typically stated as reproduction for personal gain, not performance for personal gain.