A system to monitor heart sounds, such as to detect a worsening condition of heart failure decompensation. The system comprises a medical device that includes an implantable multi-axis heart sound sensor, operable to produce, for each of at least two nonparallel axes, an electrical signal representative of at least one heart sound, the heart sound associated with mechanical activity of a patient's heart. The device further includes a controller circuit coupled to the heart sound sensor. The controller circuit measures components of the heart sound that respectively correspond to each of the axes.