really? what do you use to do the encoding??
every technical document I can find says maximum possible bitrate for an MPEG Audio Layer III (MP3) file is 320kbps
https://www.apple.com/au/ipod-shuffle/specs.html
http://www.mp3-tech.org/programmer/docs/mp3_theory.pdf section 4.4 says "The Layer III standard defines bitrates from 8 kbit/s up to 320 kbit/s,"
even wikipedia does not break with this but simply states all possible bitrates up to 320kbps
MP3 - Wikipedia, the free encyclopedia
it does add this proviso at the end of the section "Non-standard bit rates up to 640 kbit/s can be achieved with the LAME encoder and the freeformat option, although few MP3 players can play those files. According to the ISO standard, decoders are only required to be able to decode streams up to 320 kbit/s." I would caution that using any bitrate above 320kbps for a layer III audio file is not advisable as it may not play back properly on all systems
I did find a reference to 384kbps in this one
MP3 File Format Specification but it is directly related to the layer II codec