The earcanal mechanical deformation induced by the temporomandibular joint movement constitutes a promising source of energy to power in-ear devices (hearing aids, communication earpieces, etc.). The large morphological variability of the human earcanal and its intrinsic dynamic characteristics - with displacement frequencies below 1.5 Hz with average volume variation of 60 mm³ - motivate the development of non-conventional dedicated energy harvesting methods. This paper demonstrates the concept and design of a modular hydraulic-piezoelectric self-actuated frequency up conversion micromachine for energy harvesting. The mechanical energy is conveyed using a liquid-filled custom fitted earplug, which can be considered as a hydraulic pump. A dedicated hydraulic circuit drives two micro-pistons (MP) while ensuring the impedance matching between the earplug available pressure and swept volume and the MP required displacement and force. These MP actuate a mechanical oscillator associated to a piezoelectric transducer allowing the low frequency mechanical excitation to be efficiently converted into electric energy through frequency up-conversion. An innovative mechanical feedback selects the actuated MP depending on the mechanical oscillator position. By doing this, each jaw motion can be harvested. A complete theoretical multiphysics model of the machine has been established for the design and evaluation of the potential of the proposed approach. Global analytical and refined FEM approaches have been combined to integrate the fluid and mechanical behaviors. Based on simulation and preliminary experimental data, the harvested energy is expected to be 8 µJ for one jaw closing, with a theoretical 40 % end-to-end conversion efficiency.