curl post audio data with rate limit
with -H "Content-Length: 9999999" you say that your audio file is exactly 9999999 bytes long (roughly 10 megabytes), but curl reports that your file is 17456 bytes:
* upload completely sent off: 17456 out of 17456 bytes(roughly 0.02 megabytes), so either your Content-Length header is wrong (that's my best guess), or the program feeding your audio file to curl is faulty, closing stdin prematurely.
either fix your Content-Length header, or fix the program feeding curl's stdin, hopefully that should send the entire file intact.
EDIT: oh, seems that server can't handle Expect: 100-continue, to disable that header, add the argument -H 'Expect:'
(an empty Expect header will make curl omit the header entirely, instead of sending the header empty)
... but to answer the question in the title, yeah that's the --limit-rate argument.