Get a proper PSU, preferably real transformers or battery, build a 4 diode rectifier and proper caps on secondary, then use classic 7805, 7809 whatever to generate the needed voltages... screen lights are dimmed by PWM, so that is your buzz. The other noise may be a ground loop. Make star shaped grounding, center point should always be near the PSU. Try to split digital and audio ground, though I do not really see through what you do with the 555s and how you connect a keyboard there. If you could share a scribbling of your circuit it can maybe be said more precise.