If you have 100% in everything, collected all Secret Reports, and have gotten to at least 10000 (or 65535) battles (or points), at the end of the game you will see the Composer with glowing angel wings, instead of just the Producer with his angel wings.
If you get all items, all pins, complete the noise report and have played over 150 hours, you get a Cell Phone item that gives +99 attack, +99 defense and +9999 HP.
You can let Beat die and later get Rhyme instead. Also, the Beat noise form is a squirrel that kills anything.
If you get in 100,000 battles, the next battle will be against Ansem from Kingdom Hearts.