An apparatus and method for audio conversion is provided to upgrade the resolution of transmission frequency of an FM (frequency modulation) transmitter and reduce the size of the FM transmitter by applying frequency coarse tune and fine tune. The apparatus comprises a digital FM modulator, a digital frequency synthesizer, a signal converter, and an analog frequency converter. The digital FM modulator modulates a digital audio input signal into a first digital audio signal. The digital frequency synthesizer converts the first digital audio signal into a second digital audio signal, whose frequency is determined according to a first frequency conversion parameter. The signal converter converts the second digital audio signal into an analog audio signal. The analog frequency converter generates an audio transmission signal with a predetermined frequency according to a second clock signal and the analog audio signal while the second clock signal is generated according to a first clock signal.