We present an ultrasonic study of relaxor-based solid solution (1-x) PbFe2/3W1/3O3-x PbTiO3 (0.25 <= x <= 0.35) located within the morphotropic phase boundary (MPB). The longitudinal ultrasonic wave velocity and attenuation have been measured by the ultrasonic pulse-echo technique at a frequency of 10 MHz in a wide temperature range from 200 to 800 K. Attenuation peaks have been observed at characteristic temperatures, namely, at Burns temperature T-B and the additional distinctive temperature T*. To explain the experimental results, we consider a modified polar nanoregions (PNRs) model taking into account fluctuation corrections of ultrasonic wave velocity and attenuation in the vicinity of both T-B and T*. The obtained results confirm the dynamic nature of MPB effects which determine the physical properties of relaxor-based solid solutions.