The Magic of the Song

Sometimes The Tree shows itself up in the dreams of different people.
Sometimes it is calling out to them, choosing them to act as its heroes.
Sometimes it sings the songs of unknown origin.
At first, people just saw that as some kind of blessing from The Tree, a kind of good luck charm for the rest of their life.
That was until someone tried to sing the song they heard in their dreams and the music let them change the world around them.
When they did that, the notes they have sung interacted with the world surrounding them and changed it.
A few years later, someone else who has heard the song in their dreams decided to record it on a dictaphone and play it.
To their surprise, as long as it was them that played the song, its effects would still show up, even if they recorded someone else singing it.
Anyone who has heard and uses the magic of the song is called a Songster.
There have been created many different guilds that study and spread the songs.
Some even try to artificially induce the dream with The Tree singing in it to create more Songsters, be it in good or bad faith.

Effect

Every song is different in its effects, sometimes they can create weapons or balls of fire, sometimes they heal the wounds or fill someones' body with energy, and sometimes they make the crops grow faster.

There is a guild of the Song Recorders, that, as their name says record every type of song that has been heard and describe their effects.
So long, they have described over 137 different songs.
As it has been noticed, the song that is revealed in someones' dream is usually the song they need the most in their life

Source

Every song comes from The Tree, either in the dreams of the people and sometimes the sound can be heard from The Tree itself.
It is also known that the proximity of The Tree gives the songs their power, as the spells sung close to it, work almost always and create bigger, most wonderful effects, and those sung near the border of The Trees' area of influence work rarer and on a way lesser scale.

Effect Casting Time

Sometimes it's a few notes, sometimes whole hymn needs to be sung before it takes any effect.

Range

As far as the sound or someones' voice can reach.

Level

Simple songs are often few notes long, so almost anyone who isn't completely tone-deaf could hum them.
Hymns, however, can require years of training and the voice of professional singer for them to work.

Applied Restriction

Only someone who has heard the song before can sing it or even use it on any recording device.
Additionally, they have to hear it first-hand, from The Tree itself, so others can't just sing or hum it to them.
Anything that stops someone or his device from making any sound stops the magic.