As an introduction to this entry, I'm stealing someone else's introduction. Namely, RadioLab's introduction to their episode "Musical Language." If you want to hear what I'm talking about, click that link and listen to the first five minutes. (Good luck stopping at five minutes) The whole thing is interesting, as RadioLab always is, but that's the part that's relevant to what I'm going to say.
Okay I know you're probably too lazy to click the link, so I'll sum it up. Psychology of Music Professor Diana Deutsch was cleaning up the audio for a CD of audio illusions she was making, and she had a phrase on loop. A part of a spoken sentence in which she explains how sounds can sound weird: "The sounds as they appear to you are not only different from those that are really present, but they sometimes behave so strangely as to seem quite impossible." The bolded phrase was looping in the background as she walked away, left the room to make tea, and after awhile, thought she heard music, like someone singing, before realizing it was that looped phrase of her own speech. On the show, they loop it for us a bunch too, and you start to hear it. Kind of like how when you say a word over and over and over again, until it no longer sounds like a word, but just a weird sound. However, this is with a few words, and it turns into song instead of meaningless noise. Naturally, as we talk, our pitch rises and falls, and has meaning or emotion or just syntactic information encoded in that intonation. But an ordinary sentence won't sound like music, it just sounds like speech. So what happens when it's repeated? It transforms somehow. While listening to the show, you get to the point where the phrase is stuck in your head like a tune, and when the full sentence is heard, that one phrase now sticks out as though she is "bursting into song," which totally didn't happen the first time you hear it!
So what's up with that? Well I don't claim to know the mechanism going on in our brains or ears or whatever. I'm just here to present my own theories.
I call it The Isengard Effect, and when I am published and there are endless studies and hypotheses about this phenomenon, I hope you remember that I was the first to give it this (rather silly) name.
Before I explain why I call it this, I have to paint you a picture of a time and a place long ago. The time? Circa 2005, CE. The place? The internet. Y'see, back in the day, when YouTube was in its infancy (yes, YouTube was created in 2005. Yes, you feel old now that I remind you that that was nearly a decade ago), fun times were to be found online in places like Albino Blacksheep, a land of flash videos and animations that were as inane as they were impossible to pause. With treasures like The Llama Song, Numa Numa, and Badgers (with a side of Mushroom and some Snake), you could be trapped for ages (mostly because the videos looped forever and ever if you let them). I could recite all the words of any one of these videos off the top of my head even now, even though they made no sense (is that how it's told now? is it oh so old? is it made of lemon juice, doorknob, ankle, cold?).
Mashed Taters and They're Taking The Hobbits To Isengard, were two such gems. (links are to youtube, because I assumed your modern sensibilities would prefer the ability to pause them) They were just clips from Lord of the Rings movies stuck together and remixed, either the audio and video together, or the audio with strange animated potatoes dancing and getting smashed (not drunk, just mushed like potatoes are prone to do) with a catchy electronic beat behind it all. The pitch and speed of the audio (just spoken lines) were left alone, and it was usually just repeated and alternated to the beat. So it wasn't like modern speech-to-song videos, where the news or Bob Ross are autotuned along with a melody or whatever. Okay, so just a normal, nonsensical flash video fossil from the pre-youtube era. Why am I telling you all this? Because after hearing these songs, some part of your brain is changed forever, namely, the part that processes the relevant lines when you watch the original movies. Not just "heh that line was in that silly song video from years ago," more like "whoa how did I never notice that line was sung before?" Some of the lines I'm referring to are these: "They're taking the hobbits to Isengard!" "Tell me, where is Gandalf, for I much desire to speak with him" "Boil 'em, mash 'em, stick 'em in a stew." When I hear these even in the context of the movie, it sounds like Legolas or Elrond or Sam is singing. That's why I dub it the Isengard Effect.
So if it's so easy to make normal words and phrases sound like music, just by repeating them, without having to change them at all, what is the difference between speech and song, language and music? Obviously, they are related already; they (usually) both have words and sound! But usually we don't think of singing and speaking as sounding the same. (Rap and beat poetry somewhat blur the lines) They do share a lot of audial characteristics though. Speech is far from simply written words, out loud. Pitch, intonation, and rhythm are very important in speech. They're part of how we can tell the difference between sarcasm, jokes, and normal "honest" speech. You can tell where the ends of sentences are based on the pitch pattern. I could say "I'm not ready..." and depending on whether my pitch went downwards at the end of "ready" or stayed steady, you would either think I was finished, or you'd expect some more words after it. Questions go up in pitch at the end, so that you can hear a difference between "You're leaving?" and "You're leaving." A change of stress (also called accent) on a specific syllable can change the meaning or part of speech of the word (I give you a present, I present to you a gift) and a change of stress on a word in a sentence can change the meaning of the whole sentence (I didn't say she stole my money vs. I didn't say she stole my money, and so on. You can actually make 7 different sentences out of these words this way without changing anything but the way you say them). These are called lexical stress and prosodic stress, respectively, if you're interested. There are also tone languages (English is not one) such as Mandarin, where the pitch, or tone, of a word can change the whole word, not just the shade of meaning. A common example word used is the syllable "ma" which can mean "mother" "hemp" "horse" or "scold" based on the tone you use to say it (you can see a demo here, it only takes a few seconds to hear the difference).
And that's just a brief and shallow summary of the almost musical qualities of ordinary spoken language. So I guess that somewhat answers the question "What is the connection between language and music?" but it still doesn't quite explain why it's so easy to make our minds, well, change their mind about whether an utterance is one or the other, when it usually seems more clear cut.
I don't really have a conclusion or realization to end this with. I suppose I'm just encouraging you to think about language a little more, and get a little excited about it like I do. When you listen to your teacher, friend, parent, or whoever speaking, contemplate how close they are to serenading you. Think about how a typed sentence has only a fraction of the meaning, tone, and other information that a spoken one does, and take that into consideration when you troll the intertubes. Sit down at a party and listen to all the conversations washing over you, duets and trios and quartets that require the musically trained (or at least language-music trained) ear to comprehend, and marvel at how little effort it takes for you to do so.