One of the things that I see frequently on this board is that Muslims attack the Bible and defend the Qur'an by referencing the supposed unknown authors of the Bible while there is the assertion that one can trace the Qur'an directly back to Muhammad (pbuh) in a clear and unbreakable line. But that raises my question. So what?
While there are many reasons we Muslims constantly reference the authenticity of the Qur'an and the saheeh ahadith, one of the most important reasons is because if God is going to send a message, then one needs to be certain that human beings did not toy around with that message throughout time. Especially if it's going to serve as a building block for your life and worship. This alone does not prove it came from a Divine source, but it does make a good case in support of such conclusion when using additional evidences and proofs.
How does tracing the Qur'an back to Muhammad get one back to the author of the Qur'an? For Islam also makes it clear that Muhammad is not the author, but Allah is.
Tracing it back, in addition to what I mentioned above, allows us to conclude that it came from the most trustworthy source. Now, this alone is not used as proof for the Qur'an's Divine Origin. Rather, it is used as supplementary evidence. So summing with the first point I made, we can verify that this Book is unchanged over 1400 years, and it was revealed to a very trustworthy person (as verified by authentic hadith). Again, these two points alone do not prove Divinity, but they build to make a strong claim when paired with other evidences.
Ultimately, what, besides Muhammad's own testimony that he received the Qur'an from Allah or the personal opinion of many that it is such a beautifully written book that no human could have composed it, do we have to substantiate that Allah is indeed the author of the Qur'an?
So now we get to the meat of the matter, so to speak. We have the above two evidences as strong reason to believe that we have the book in its original form and that it came from a very reliable source. But we haven't even touched on what the Book is about or what it says.
This is a matter that is discussed in numerous threads, but to sum up the main proofs for the Divinity of the Qur'an based upon its contents:
1) Looking through the history of the Arabs we know they were a people engaged in prose and poetry and that the Qur'an came at the height of this period. It defied categorization into either poetry, prose, or both: it came down as something unique altogether. The Arabs could not challenge it or produce something like it. This is still evident today, as you can see many laughable attempts to challenge the Qur'an, many of which blatantly copy whole passages from the Qur'an. It also speaks with such confidence about things such as predictions of the future, describing the natural world, and historical events not known at the time. There is also amazing consistency and connection between the chapters of the Qur'an, and given that it was revealed of 23 years like a jigsaw puzzle is quite a feat in itself.
2) I'm not going to get into the scientific proofs in detail (because once those are mentioned the thread will probably spiral out of control), but suffice it to say there are some very strong descriptions in the Qur'an about the natural world that could not be made by someone in the 7th century, no less an Arab with no education or knowledge of the world outside Arabia.
3) There's a saying in the writing world: "write what you know". Usually writers who adhere to this policy end up writing a lot better than when they don't (I know because I used to write fiction quite a bit). The Qur'an is not a book we would expect Muhammad to write. He lived his life in the desert, yet it's a book that talks about so many different types of environments on earth; it talks about the sea and the bottom of the ocean, and the tops of high mountains. He was very much concerned with health and higene, yet the Qur'an makes very little mention to either, except that we need to wash before prayers and that honey is a good cure. It doesn't contain any weird home remedies for diseases or anything of the sort. It's also completely different stylistically from the hadith. I know one very difficult thing to do as a writer is to write two pieces of literature and have them expressed in very different voices. This is especially true for a man who was never a poet, or a speaker of any kind before his prophethood.
4) This Book single-handedly transformed the Arabs from a war-mongering people in the desert that not even the Romans cared to conquer, to a group that would conquer an empire larger than any the world has known for over a thousand years. It made Arabic the lingua franca of the day (much like English is today), got Europe out of the Dark Ages by sparking the Renaissance, gave women rights the West has only come to know 50 years ago, and much more.
Could this have all been accomplished in 23 years by a man who was neither a religious leader, politician, orator, or military leader before the age of 40 without the help of God?
My conclusion: Qur'an = Book from God, revealed to the Prophet Muhammad.
and slowly I realised that it had been months since I played and heard a song. Now when I listen to music I get distracted and have no affection, whatsoever, for it. I don't miss it either. Alhamdulillah Alhamdulillah Alhamdulillah. It has been said a million times before and I'd just repeat it, Music is unnecessary (like smoking) and we must realize that life is possible without it. 
