Convert opus to spx

What is the difference between .opus and .spx?

Description: An OPUS file is an audio file created in the Opus format (also called "Ogg Opus"), a lossy audio format developed for Internet streaming. It uses both SILK (used by Skype) and CELT (from Xiph.Org) codecs and supports variable bit rates from 6 kb/s to 510 kb/s.; 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.