Sherlock:
Everyone knows that Sherlock plays the violin. But what they don't know is that he is also an excellent singer. He also never sang but once you heard him singing to himself when he thought you and John were away.
John:
When John was in school he had learned to play the clarinet. His parents had even bought him his own after a while. He had liked it at the time but now he was kind of embarrassed about it. You insisted that it wasn't embarrassing and once even convince him to dig out the old instrument from his closet and play it for you. He was rusty but still good. It's one of those things that you love you know about him because no one else does.
Mycroft:
While his brother learned to play the violin Mycroft was forced to endure many years of piano lessons. He was a prodigy as his brother is however that does not mean that he enjoyed it. Still from time to time if he has the time he will sit and twiddle on the grand piano in his sitting room. It's not often that he does but you still love to sit and listen or if you are feeling adventurous the two of you will play a duet with the instrument you play.
Greg:
A little known fact about Greg is that when he was at university he was in a band. Him and some of his friends had thought up the idea one day and gave it a go. They were actually pretty popular on campus and played several shows at local pubs. They had even made a CD of themselves that Greg still has. He was the basset and the secondary vocalist. You actually like the music and will play it sometimes on long car rides with him. (sometimes you get into the music and that's how you end up getting lost.) Greg always rolls his eyes when you play it. But still has fun telling about all the stories behind each song and some of the crazy nights him and his band mates had.
Moriarty:
If he had chosen Jim could have been a composer. He plays several instruments and it givened in all musical endeavors. However Jim prefers a more interesting life than that. But he will still play for you from time to time upon request. His favorite instrument to play in the cello. He always becomes absorbed in it. He likes how sturdy it is and that he can feel the vibrations through the whole of his body when he plays. You love to listen to him play and watch and his mind goes completely blank of anything but the music.