First of all, let me say that I believe in Physics and have a very straightforward view of the universe.  I define a "sound" as vibrations caused by the interaction of a physical event with the surrounding air.  As such, it would not matter whether there was a tympanic membrane present to translate it into nerve signals for a brain to process.

Philosophy has always seemed to me as something to occupy Freshmen until they grew up enough to see the real world looking over their shoulders, and a kind of  mental masturbation, along the lines of  "Hey, dude, what if <insert any idea occurring to a drunk or stoned mind>"

Then again, some people seem to take it seriously, and I owe the following to Simone Hochreiter, who found the following at
www.whitworth.edu/Academic/Department/Core/Classes/CO250/UK/Data/d_berke.htm

GEORGE BERKELEY (1561-1626) 


BIOGRAPHY 

George Berkeley was born near Kilkenny, Ireland, and, although an 
Anglican of English descent, he emphatically considered himself to be 
Irish. He studied at Kilkenny College and in 1700 went on to Trinity 
College, Dublin. There he read Descartes, Newton, and Hobbes. In 1707, 
he became a Fellow of the College and was ordained in the Anglican church. 
The next six years were to be the most philosophically productive in his 
life. In 1709, he published his New Theory of Vision, and in the following 
year his most important philosophic work, A Treatise Concerning the 
Principles of Human Knowledge. In 1711, he wrote Discourse on Passive 
Obedience. Two years later, he published a more popular exposition of the 
doctrine of his Principles in the form of Three Dialogues Between Hylas 
and Philonous.

In 1733, he published Alciphron, or The Minute Philosopher, against the 
freethinkers (agnostics), and in the following year The Analyst, a 
criticism of Newton. That same year, he was made Bishop of Cloyne. For 
the next eighteen years, he energetically served his remote, poor diocese. 
Among the works he wrote during this period are The Querist (1737), which 
used questions to propose public works and education as remedies to the 
crushing poverty he observed, and Siris (1744), an unusual work dealing 
with the medicinal value of tar water. In 1751, he lost his eldest son, 
and the next year he moved to Oxford, where another son was beginning his 
studies. On January 14, 1753, Berkeley died suddenly and he was buried at 
Christ Church, Oxford. 

BASIC THOUGHT 

Like Locke before him, Berkeley accepted the empiricist doctrine that all 
we can know are ideas and that ideas come from perception or reflection. 
But Berkeley saw a problem in Locke's assertion of an external world of 
material "substances" giving rise to perceptions. If all we can know are 
ideas, how can we know there is a world "out there" giving rise to our 
ideas? Locke had said that the primary qualities of an "external object" 
(such as extension and solidity) are "utterly inseparable" from the objects 
themselves, whereas this is not the case with secondary qualities (such as 
color, taste, etc.). But again, asked Berkeley, how can Locke know this? He 
cannot get "outside himself" to see which of his perceptions are actually 
a part of objects "out there." Berkeley concluded that Locke's philosophy 
will lead to skepticism, whereby we must admit that we cannot really know 
anything about the world "out there."

To avoid this skepticism, Berkeley made the radical claim that there is 
no "out there," or, more precisely, there is no matter. Berkeley's 
position, which is called "idealism," can be summed up in his famous 
phrase "esse is percipi": to be is to be perceived. What we call "bodies," 
or physical objects, are simply stable collections of perceptions to which 
we give names such as "apples," "trees," and so on. These collections of 
perceptions have no existence apart from a perceiving mind. The answer to 
the famous conundrum "If a tree falls in the forest and no one hears it, 
does it make a sound?" is that if no one is perceiving it, it not only does 
not make a sound, the tree does not even exist!

Does this mean that trees go out of existence when no one is left in the 
forest to perceive them and that they come back into existence when someone 
enters the forest to perceive them again? It would seem that Berkeley must 
accept this odd conclusion were it not for one important point: God never 
leaves the forest and God is always perceiving the trees. By always holding 
all collections of perceptions in the divine mind, God ensures their 
continued existence and the perceived regularity in what we call "nature." 
This point has been classically formulated in the following limericks:

There was a young man who said, "God,
Must think it exceedingly odd
If he finds that this tree
Continues to be
When there's no one about in the Quad."

REPLY:
"Dear Sir: Your astonishment's odd:
I am always about in the Quad.
And that's why the tree
Continues to be,
Since observed by, Yours faithfully, God."

Berkeley saw his philosophy as a common-sense attack on the metaphysical 
excesses of medieval Scholastics, Continental Rationalists, and even 
fellow empiricists such as Hobbes and Locke. Although Berkeley understood 
his philosophy to be common sense, his readers came to different 
conclusions. One prominent physician of his day claimed Berkeley was 
insane. The great Dr. Samuel Johnson dismissed Berkeley's ideas with 
his famous "I refute Berkeley thus" and then he kicked a rock. Of course, 
this did not refute Berkeley at all. It only proved Johnson had not 
understood Berkeley's point. Berkeley did not claim the non-existence of 
stones or that kicking a stone will not produce sensation. He claimed the 
rock did not exist apart from the perception of its solidity or the 
perception of pain when struck, and so on. An oft-repeated epitaph 
summarizes the general reaction to Berkeley: "His arguments produce no 
conviction, though they cannot be refuted."

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And to explain further the remark about Dr. Johnson above I found the following at
http://www.samueljohnson.com/refutati.html

57. Refutation of Bishop Berkely
After we came out of the church, we stood talking for some time together 
of Bishop Berkeley's ingenious sophistry to prove the nonexistence of 
matter, and that every thing in the universe is merely ideal. I observed, 
that though we are satisfied his doctrine is not true, it is impossible to 
refute it. I never shall forget the alacrity with which Johnson answered, 
striking his foot with mighty force against a large stone, till he 
rebounded from it -- "I refute it thus."
James Boswell: Life of Samuel Johnson book 3.