Conventional vibration sensing systems, equipped with specific sensors (e.g., accelerometer) and communication modules, are either expensive or cumbersome in deployment. In recent years, the community revisits this classic topic by taking advantage of off-the-shelf RFIDs. However, limited by lower reading rate and larger wavelength, current RFID based solutions can only sense low-frequency (e.g. below 100Hz) mechanical vibrations with larger amplitude (e.g. (>) 5mm). To address this issue, this work presents TagSound, an RFID-based vibration sensing system that explores a tag's harmonic backscattering to recover high-frequency and tiny mechanical vibrations accurately. The key innovations are in two aspects: harmonics based sensing and a new recovery scheme. We implement TagSound with USRP platforms. Our comprehensive evaluation shows TagSound can achieve a mean error of 0.37 Hz when detecting vibrations at frequencies below 100Hz, and a mean error of 4.2 Hz even when the vibration frequency is up to 2500Hz.