My monitor has built in speakers but I can't seem...

Sounds like an inverter problem. If the power supply is internal replace the capacitors coming out of the voltage regulators. These are usually 1000mf and/or 470mf. If that doesn't do the trick look up the numbers on the 8-pin chips that drive the secondary transformers on the inverter to see if they are getting the right voltages.
Good Luck!