I can be still, but never is the air;
Unless my strings do not feel horsehair.
A stringed instrument played with a bow;
There are typically "between 150 and 200 hairs from the tail of a horse for a violin bow."
The instrument may be held still but its strings when vibrated by the bow will vibrate the air, producing sound.
I am a combination of maple and spruce.
A violin's soundboard is usually spruce, and its ribs, back and neck are usually maple.
Most find difficulty when I am first introduced.
This is a difficult instrument to learn to play.
My modern relatives have been around for years.
Probably referencing the viola, whose present form has been around since the 18th century. The etymology of the names "violin" and "viola" are rather intertwined, though, and run back well before then.
Our sounds can be full of joy and tears.
Violins can evoke a range of emotional responses in how they are played.
I may be different, as I have a beauty spot;
And bows reach my height, but for others, they do not.
There's a so-called "sweet spot" at a particular spot on the strings where they are most resonant. It's in a different spot for different instruments.
I am said to be melodious by most and not few.
Guess what I could be, and your guess might be true.
I hope so!
My neck is vaguely longer than my waist.
Though this is revealed only when I am uncased.
The "neck" and "waist" are parts of a violin's structure, and indeed the neck is a bit longer than the waist. Violins should be kept in a case when not used, so of course you wouldn't see either the neck or the waist until you removed it from its case.