Observations of plate waveguide structures through body-wave dispersion signals have in recent years become an important tool for assessing the characteristics of subducting plates. The subduction zone of Sumatra has generated multiple catastrophic megathrust earthquakes and tsunamis in the past, such as the event of 26 December 2004, making detailed investigation of the subducting process highly significant. In this region, the Australian oceanic crust is deeply subducted beneath the Sunda continental plate, forming a distinct plate waveguide structure. In this study, we analyze intermediate- to deep-focus earthquake signals recorded by permanent broadband seismic stations operated by the Indonesia Agency for Meteorology, Climatology, and Geophysics (BMKG) to investigate the properties of the subducting oceanic plate in this area. By selecting seismic data from stations distributed around the subduction zone and comparing records from forearc and inland sites, we confirm that seismic records showing high-frequency P- and S-wave delay phenomena occur only at forearc stations. Seismic records were converted to displacement, instrument responses were removed, and narrow-band filtering and Hilbert transforms were applied to obtain smoothed dispersion curves for examining body-wave dispersion. Further analysis reveals that delays in P- and S-wave arrivals occur in the 1–9 Hz high-frequency range, with corresponding 0.5 Hz P-wave first-arrival energy showing phase delays of 2–4%. We infer the presence of a low-velocity waveguide structure in the Sumatra subduction zone, located just above the subducting plate, characterized by low attenuation, with a thickness of approximately 1.8–4.1 km and a velocity of about 7.9–8.1 km/s. Compared with high-temperature, high-pressure rock properties, we propose that the conversion of gabbro to eclogite at depths of ~100–250 km within the subduction zone is incomplete, and that the region may contain dry eclogite, gabbro, their mixtures, and hydrated structures.