Does anyone know if the MP3 player of the attachment changes the bitrate for steaming or if it actually streams the original?
if you upload at least 1080p video to YouTube, and the user watches as HD, that was supposed to be the best quality, but YouTube does it’s own thing to the entire video stream and you can be sure it’s been squashed in some manner if your upload had uncompressed or FLAC audio. I suspect even 320kbps does not survive.
If it’s a detailed mix you might consider using something like Dropbox or similar and upload a non-lossy format. Or, SoundCloud does offer HD (at some cost), which I think will guarantee your 256k and maybe 320k get through. I usually upload 320k there because the embedded player is widely supported and I think you’ll get more listens that way.