The AY-3-8910 PSG is capable of playing audio without an Intellivoice, as it can on other platforms like the MSX and Spectrum. It’s not high quality, and an Intellivision ROM cartridge cannot hold more than a few seconds of audio, but it does work! 
Intv Prime has produced a tool call "Sonum Vox" that will convert audio to an IntyBASIC program, using knowledge from Arturo Ragozini.
Óscar Toledo G. has published code to play waveform without Intellivoice in his book "Advanced Game Programming for Intellivision".
Media Links:
media0101.intvprime.com/ba4ef/intellivision-echo/Game-Audio-Samples/Aliens-Game-Over-Phrase.rom
Source Links:
map.grauw.nl/articles/psg_sample.php
atariage.com/forums/topic/253650-voice-without-intellivoice/