Military musicians

Military musicians have as many know been used by several different warlords in many historical periods, for instance by the Romans, by various Turkish armies and later also by many western and central European armies too, in order to simplify distributing orders to the regiments.

I've got some questions about these often forgotten soldiers and their history, if anybody knows the answers:
1. who were the first known to use musicians in battle, and when was their first recorded usage of it?
2. how did the romans distribute their instruments and how were the signals built up? I've heard they had different instruments, but how exactly did the different order signals differ from each other, and how did they identify which cohort/cohorts to give the orders to? Did the romans also put musicians in the cohorts too in order to reply to the signals?
3. someone said the Finnish composer Jean Sibelius used old Finnish military battle signals in one of his musical pieces. Is it true, and if so, which piece?
4. how did the signals typically sound in the Dark ages? (I had in mind using the answer to this particular question to if possible compose a battle music piece for the AoVaF mod including military signals)
5. when were military musicians as tool in battles abolished, and why? I believe Napoleon used them, so I guess it was somewhere betwen the Napoleonic and First World War era, probably connected to the development from important single field battles to a more frontline oriented combat, or am I wrong?

Did the cavalry in World War one not use buglers?

About your second question -
The knowledge about the tubicines (trumpeters) and the cornicines (horn players) who served in the Roman army is very little .
The Roman king Servius Tullius gave them , in his constitution , 4 centuries in the comitia centoriata , that is , the assembly of the soldiers in the Roman army . Early Roman instruments were borrowed from the Etruscans including a variety of tubas with straight, bent and coiled shapes . Military musicians were senior centurions (principales) . Their music performed many tasks . Trumpet signals were used to :
Sound the alarm
Give the signal to attack
Give the signal to retreat
Formation changes in battle
Changes of the watch
And as accompaniment to the march.
"The sophistication of their battlefield tactics, the flexibility of their formations during battle and the relatively high rank of their trumpeters and horn players suggests that their musical signals formed a highly complex and important military communication system."

As , I have said , we know nothing more about them .

to question 1
the egyptians have been shown in there battle reliefs to have used a high squeeky horn to give commands. though it is believed that they had the limited use of simply attack and retreat.

the romans way of using the horns was complecated but effective. you asked out adressing individual formations well that is delt with simply enough, the 3rd century in the first legion would hear three long and low sounds and the centurian would know that there are incoming orders. once those orders were givin he would have either a whistle or a trumpeteer by his side to give that same order to the legionairs and the squad leader would make sure they all know the command. this was also a reason that there helmets were developed to have open ears, good hearing.