In this paper, we study the spatio-temporal variability of the sound speed in the Bay of Bengal (BoB) estimated from the Argo observation data during 2011-2020. We perform domain-wide and region specific analysis of the sound speed structure and identify the regions and times of higher variabilities. The domain-wide spatio-temporal variability in the sound speed is maximum in the thermocline layers near 110 m depth. This variability is smaller at around 35-40m depth but increases in the surface layers. The regions of higher temporal and spatial variability vary with depth and time. In the surface layers, the variability is large in the northern part of the Bay but in the subsurface and the layers underneath, it is large along the entire western boundary from the north to south. Due to the combined impact of temperature inversion and the positive salinity gradient, the northern BoB experiences a significant positive vertical gradient in sound speed above the sonic layer depth (SLD) during the postmonsoon and winter periods. This gradient supports strong surface ducting and formation of the shadow zone below the SLD.