This article confirms my point of view. There is no objective truth.
The first used example is already a bad one. "What day of the week is it today?" has as many answers as calendars with different week systems. 220 years ago, the answers in France and England would have been different.
Other easily to debunk sentences:
"Gravity is the same no matter what the historical context or what your religion is."
But not necessarily in a time-space anomaly while travelling at the speed of light.
Only if assuming a counting system of 1, 2, 3, 4, 5...
If it was 1, 2, 3, 5, 4, 6, ..., then 2+2=5.
"Communication relies on there being an objective truth.".
Nope, it relies on convention. Like the used language.
Not to be misunderstood: Seeking truth is worthwile. But it will never be an absolute or objective truth. It will be truth relative to those who find it, and to their living conditions, and their field of research.
Even the other example you provided, about the speed of light, is not an absolute/objective truth. It's what we believe to be true; but only as long as no physicist finds out stuff Einstein didn't know about. Maybe under some particular time/space conditions, light travels at a different speed.
And to go back to the subject of piano playing: It involves the pianist, the instrument, the room (including listeners), and the music to play. These components need to work together in order to produce fine art. And I don't think there is one universal way to do it.