Ultrasound-assisted metal additive manufacturing (U-FAAM) has emerged as a promising in situ melt-pool regulation strategy for addressing the challenges of conventional metal additive manufacturing, including process instability, defect formation, and microstructural anisotropy. This review systematically summarizes recent progress in contact and non-contact ultrasound-assisted metal additive manufacturing from the perspective of acoustic energy delivery pathways. The interactions between ultrasonic excitation and melt-pool behavior, thermal transport, solidification-front evolution, defect formation, microstructural transformation, and mechanical performance are comprehensively discussed. Contact ultrasound approaches transmit acoustic energy through solid media. Cavitation can occur only when the melt-side acoustic pressure reaches the required threshold; under such conditions, cavitation-associated effects may act with acoustic streaming and inertial melt flow to enhance mixing, grain refinement, and defect mitigation. Non-contact ultrasound approaches provide improved geometric adaptability and mainly influence melt-pool evolution through acoustic streaming and cyclic pressure fluctuations. The relationships among ultrasonic input conditions, melt-side responses, solidification behavior, and final properties are critically analyzed. Furthermore, current limitations associated with acoustic energy attenuation, coupling efficiency, process scalability, and quantitative characterization of ultrasonic effects within the melt pool are highlighted. This review synthesizes current evidence for ultrasound-driven melt-pool control in the investigated fusion-based AM systems, particularly DED/LDED and WAAM, while recognizing the limited evidence for LPBF.