The present invention provides an audio-data transmission system including a channel encoder, a data modulator and a speaker. The channel encoder is utilized to receive and encode a bit signal and generates an encoding signal. The data modulator is utilized to receive the encoding signal and modulates it to an audio signal whose frequency is on a frequency band. The audio signal includes at least four different sub-audio signals. The frequency band includes at least four sub-frequency bands with different frequencies. The frequencies of the at least four sub-audio signals are respectively on the at least four sub-frequency bands. The speaker is utilized to transmit the audio signal including the at least four different sub-audio signals.