Why do the brakes squeak on 2001 Nissan Sentra GXE when you release them?

Disc brakes by their very nature can be noisy due to the high heat and pressures they work in. As you depress your brake a great deal of heat is being generated and holding the brake keeps these parts from vibrating however, as you release the brake , there is enough play to allow this vibration. Chances are your brake rotors have enough wear so they can no longer absorb this heat like they could when they were new. If it is troublesome I would replase the brake rotors and pads.