You can play an audio to the participant of the call using either the play API or the play XML.
The audio file is obtained from a remote URL. Plivo supports mp3 and wav file formats.
Preferences for MP3 & WAV file which can be used are as below,
MP3 file should be with preferences: MPEG ADTS, layer III, v1, 128 kbps, 44.1 kHz, JntStereo
WAV file should be with preferences:- WAVE PCM, 8,000 Hz 16 bit or 11,025 Hz 16 bit PCM