At last, the long awaited "Magical Music System" has arrived! I've worked really damn hard on this, even if it doesn't show. I have gone through at least three itterations, including one where you had to specialize by music *genre*! Despite all that, I finally got it out. At a basic level, this is a tool-kit for bard characters to make spells interesting, by making them compose music, which has effects through character point values.
Now, there is one new addition to character sheets with this, but you don't need to track it too often. It is called MP, or Musicality Points. It's supposed to represent your character's creativity, will to work, and everything else like that. It is ten, generally, though you can change it for +/-2 points. It has a maximum of your Will, and a minimum of five. They are explicitely used in the "composition process", not for spellcasting. MP restore at a pace of one per day. (Though if you want you could have an increased rate of this, with a low-cost advantage, at your GM's discretion.)
Onto the juicier bits, now. To compose music, first determine what you are attempting to compose. Come up with a title that encapsulates what you are going to do with your magical song. For example, a song that heals a companion could be called "Song of Restoration". Then, you must make a roll against your Musical Composition skill to determine your points available for your composition, as well as declaring how many of your MP you are expending.
If you fail this roll, you get refunded half your MP (minimum of one), but you cannot try a composition until tomorrow. Otherwise, you multiply your Magery by your used MP, to get your CMP, or Cost of Magical Piece. Despite the name, that value is the maximum of the value, NOT the current value, which starts at zero. You can then build the song's effects using traits, though work closely with your GM to determine what you can and cannot use. Afflicting a disadvantage on someone else costs points equal to the negative point value. Format the CMP score as "current(maximum)."
Some specific types of songs (like resurrecting someone that has died, making them under your control) might be banned, if it falls under a pre-existing magical college. Another example is that a song that creates a blast of fire would be banned in a society that has banned Pyromancy. Once you've made the spell, it's effects, and all else, move onto the next step.
Determining the length, cost, and skill. Length is easy is easy, as you can make it as short or as long as you want. You must make a concentration roll consisting of your Musical Skill (see third part here) plus any relevant bonuses. Cost is determined by dividing your point total by ten, with a minimum of one. Every five seconds of playing, you lose another one FP (NOT your full cost.) A critical success makes you lose only one FP, or zero if you have a cost of one FP.
Skill is just a simple determination of what you want to do. It is either singing or a specific musical instrument, or both, which means you'd roll against the lower first, add the MoS/F, then roll against the other skill. Doing this makes you lose two FP per five seconds. Continue this step for bands of players; whoever has the highest skill gets to roll with everyone else's MoS/F added onto their skill.
Example A: Sir Marvy has a song called "Strength of a Bull", with a CMP of 20(20), cost of two, and skill of Singing-15. He is currently in combat, singing for his companion Sir Benjamin. He rolls a 14 (5+5+4), meaning his song succeeds for this second. Until he breaks concentration or fails a roll at the end of the next five seconds, Harry has the trait Increased Strength 2.
Example B: Sir Marvy now sets out to make another song, called "Silence of Winter", which is attempting to make someone mute. This requires a minimum of CMP 25. He has Magery 3, expending ten MP, meaning a CMP of 30. He succeeds his composition roll, getting CMP 25(30). His song works, though the enemy must roll against their Will, just like any other spell being cast on them.
Example C: Harry is being hit with a spell called "DEATH METAL", a spell that causes someone to get a heart attack and decrease their basic move. The spell itself is extremely expensive, being cast by an evil Bard named Jaethorn. Jaethorn succeeds his Musical Instrument+Singing roll, although Harry succeeds his Will roll, negating the death. (Warning: This is a reach, as that spell is insanely powerful and might not get accepted by a GM.)