Convert vox to spx

What is the difference between .vox and .spx?

Description: Audio file encoded in Dialogic's ADPCM (VOX) format, a format used for saving human speech audio; saves digitized voice data sampled at a low rate; enables the audio data size to be small while still maintaining clarity; used by computer telephony systems.; Audio file compressed using Speex compression and saved in the Ogg Vorbis container format; designed as an audio compression format specifically for speech; compresses voice at 2 to 44 kbps and supports 8 kHz, 16 kHz, and 32 kHz compression in the same bitstream.