Despite its title, this book reaches far beyond sound and music. Really a compilation based on a variety of writings and transcripts of lectures delivered during the 1910s and 1920s, Hazrat Inayat Khan shows how sound and music are a pathway to a broader enlightenment. Granted, "enlightenment" is a charged word with more than one definition, and some will scoff at its use altogether. This book is not for them. But for those open to an honest exploration of human spirituality, this book contains a great deal to recommend it.
Khan writes and lectures from a Sufi perspective, but is deliberate about drawing parallels to other faiths, including Hinduism, Judaism, Christianity, and others besides. In fact, to his credit, he even suggests at one point that it would not be desirable for all people to adhere to a single religion, although he implies that all people should adhere to some religion, or at least some form of spiritual practice. Khan, it should be noted, defines the equally-charged term "religion" quite broadly to include all spiritual beliefs and practices.
Given its nature as a collection of separate writings, there is no central thesis, per se, to this book, but there are themes. The author posits that vibration is the source of the entire universe, both physical and metaphysical, and is able to reconcile this with modern (1920s) physics. Thus, Khan is not a science denier, although one of the flaws in this work is his penchant for pseudo-science. Vibration is then linked to the phenomena of sound (the audible) and light (the visible), which are then linked to such "vibrations" as the rhythms of the human body and nature. The reader must avoid interpreting Khan's use of such terms according to a strictly Western idiom, since his meaning when he speaks of "vibrations" is much broader than a physicist might understand it to be. This is not, however, ignorance on the part of the author, it's merely a difference in the use of language, and the attentive reader will quickly glean Khan's meanings from the context in which such terms are used.
The author also devotes quite a bit of time, especially in the latter half of this volume, to the idea of "the Word" as both the source of all things and a mystery which, when unraveled by each individual, forms a path to enlightenment. This is a challenging concept to grasp, but Khan patiently explains his meaning, often employing illustrative examples and stories which lend an accessibility to this book which is sometimes lacking in other works of Eastern philosophy. Khan is very deliberate about trying to explain concepts in the simplest, most directly-relatable manner possible, and avoids complexity and obfuscation at every turn, which in and of itself is quite a feat for a book so thoroughly steeped in mysticism.
Where Khan fails, primarily, is in his overly-zealous defense of mysticism at the expense of rationalism. This is the same error which the New Atheist make when rejecting all spirituality, but Khan is coming at it from the other side. His tone is, at times, a little too dismissive of science, and he borders on the sanctimonious when characterizing Western beliefs, practices, and culture. He also references all manner of pseudo-science, much of which probably seemed more plausible a hundred years ago than it is today. In addition, he is at times flat-out ignorant, espousing views which, a century later, are painfully misogynistic and racist. It is, thus, that I have docked this work one star, not because it doesn't contain valuable content -- in fact, this has been a game-changer for me, personally -- but because there is a fair amount of grimy bath water in which the baby is sitting.