To be an ideal candidate, seismic sensor for wireless unattended ground sensor (UGSs) applications should have lightweight, low noise, high sensitivity, and energy efficiency. Typically, to cover regions far from source, the best choice will be the coil-over-magnet geophone. Because it is usually heavier than batteries, a lighter one should replace it to further cut the weight of sensor nodes. However, currently available seismic sensors, such as micro-electro mechanical systems (MEMS) accelerometers and molecular-electronic transducers, cannot do the job since they usually consume too much energy to achieve low noise level as well as high sensitivity. This work has designed and tested a new kind of vibration sensor, the vibration-to-sound geophone, which can convert seismic waves into sound physically that can then be detected by an MEMS microphone. By using a battery as its proof mass, the vibration-to-sound geophone 1) has better sensibility than the coil-over-magnet geophone from 20 to 500 Hz and is about 58 times more sensitive at 70 Hz which is more than 60 dBV/m; 2) is very light, half the weight of the coil one; and 3) consumes no more than 726 μW which is more energy-efficient than MEMS accelerometers and molecular-electronic transducers.