The paper presents the results of acoustic studies of the melting and crystallization of Ag–Ga alloys with a silver content of 1.5 and 3 at % embedded into porous glasses with an average pore size of 13 nm. The temperature dependences of the velocity of longitudinal ultrasonic waves are measured by a modified pulse-phase method at a frequency of 7 MHz in the 200–325 K range for complete and partial cooling–heating cycles. The temperature dependences of the ultrasonic velocity showed regions corresponding to phase transitions. Significant changes in the phase diagram of the bulk alloy due to nanostructuring have been revealed. It is shown that segregates with different crystal structures are formed in the pores for alloy of different compositions.