A handheld electronic device includes a first speaker, a second speaker, and an audio controller. The audio controller includes a signal receiving unit, a frequency crossover unit, a harmonic-wave generating unit, a low frequency enhancement unit, and a signal synthesis unit. The signal receiving unit receives an original sound signal and generates a first channel signal and a second channel signal according to the original sound signal. The frequency crossover unit sets a preset frequency crossover point to filter the first channel signal to generate a low frequency signal. The harmonic-wave generating unit generates harmonic waves according to the low frequency signal. The low frequency enhancement unit generates an enhancing low-frequency signal according to the harmonic waves. The signal synthesis unit generates a first channel enhanced signal according to the enhancing low-frequency signal and drives the first speaker.