See the thing was, was that Dean loved to sing and would sing every chance he got. But, god help him, the boy could not hold a tune in a bucket. Sam had ridden in the car with him his whole life with Dean belting out every classic there was, holding back the cringe just because Dean singing was the only time it seemed he was content.
It didn't always mean he was happy though. Sometimes it was how he geared up for a moment, sometimes it was how he would wind down. Sometimes it was how he tried to help Sam through choices that had to be made and sometimes it was just so he could annoy him. So many moments in their lives were bookended by Dean's awful, out of tune singing. Def Leppard and Bon Jovi at the top of his lungs like their lives depended on it, because sometimes, it felt like it did; Metallica under his breath to calm him; REO Speedwagon for reasons that remained mysterious to both of them; Asia, until Sam begged him never to sing it again.
For the time that Dean was "away" (Sam would never call it anything else), there were rumours that Dean even karaoke'd. Sam had seen what the three b's (bars, bourbon and blondes) could do to Dean; throw in a pair of black eyes and there was no doubt in Sam's mind that the rumours were true. And very briefly Sam felt a little pity for those around him, sober singing Dean was bad, however entertaining but drunk singing Dean was a tidal wave of embarrassment for everyone. Well everyone except Dean. Dean was never embarrassed by his singing. Well except for that one time. It was a bad time. Baby was gone. Cas was gone. There was Air Supply. They never talked about it again.
They had crossed the country more times than they could count, with the bass thumping through Baby's speakers, mile after mile. Stairway to Heaven, all 8 minutes, the ear ringing 4-chord progression that was Ace of Spades, Ozzy singing about being Paranoid, anything that AC/DC ever put on tape, they all formed the basis of Sam's understanding of Dean. Everything was loud, slightly off centre and a little questionable; Dean and his music were interchangeable.
But, see the thing was, was that Dean had a secret, and Sam knew Dean had a secret. When Sam was asleep (or pretending to be), the volume came down and the singing changed. Dean, with his brash over the top singing style disappeared and in his place, in what he though was the privacy of his car another Dean appeared. And this Dean could sing.
This was the voice that Sam never heard when Dean thought he was listening. Skynyard's mirrored country twangs that came out when he was singing about being a Simple Man, harmonising with the Eagles on Seven Bridges Road, keeping rhythm on Baby's steering when he pulled into Nazareth with The Band. There were the songs that had no music in his collection so he just sang them, the songs that Dean had sung to him when he was little. It never occurred to Sam until much later that maybe Mary had sung those same songs to Dean, quiet and still, almost lullabies, soothing and sad at the same time. Sam inevitably turned his head away when Dean sang these songs alone, after the first time he had seen the tears in Dean's eyes when he sang them he never wanted to pretend he hadn't seen them again.
Dean, the inveterate gambler, had a tell. Sam hadn't noticed it for a long time until it gradually it started to show in the little things he did. The way he would run his hands over the keys of a piano when he saw one or pick up a guitar, only to check himself and put it down. It didn't mean much to Sam until he spoke to Robyn at the boys home (something else Dean had relegated to the "we don't talk about it" file), when she talked about guitar lessons with the boys. A natural she had called him, with a great ear, could listen to a tune once and play it within a few strums. She knew she was in love when he took her out on the porch and sang to her. Sam didn't know which shocked him more, Dean being in love or Dean playing the guitar. The singing he knew, of course, he had been secretly listening to Dean for years by then. The time it really hit home, however was while Dean was "away", and hit it did, like a fist to the solar plexus. Dean knew Sam was there, he just didn't know he had been there for a while. Sam had seen him sitting at the piano, his hands on the keys like he was trying to find his way in dark room; like an old lover he was trying to reacquaint with. He was finding a tune of his own, pressing a key, taking a drink, pressing two keys, taking a drink, there was a tune there, he just couldn't find it anymore. Sam watched him at the keys and wondered how he could have missed so much, and when the first two chords of Hey Jude came from those keys, all of the air was sucked clean out of him. All he could think was, please don't play it, please don't play it.
That was all a long time ago now and Sam and Dean have shared so many things since then. No more secrets they had said, had agreed. Life is too hard and too dangerous for them to have secrets from each other anymore, only trouble comes from keeping secrets. And Dean has agreed to this and is trying so hard, so is Sam, but it's not easy; a lifetime habit is hard to break. And Dean has his own room and an iPod now so can listen to whatever he likes, but in the car it's still the same classics as always, and like always, Sam pretends he doesn't know, a secret he is willing to keep. But on a beautiful sunny day, with Night Moves pumping through Baby's ancient tapedeck, Dean sings, and so does Sam, and all he can do is smile because Dean is not out of tune, Dean is singing harmony, and above all Dean is happy.
Because the thing is, when Sam hears Dean sing, that's all he has ever really heard.