make my mp3 send audio directly to the computer through usb. It seems like it would be an easy thing, but
No, this is not easy.
There are two possible options for this:
1. record and encode on the mp3 player and transmit a final mp3 file to the PC. This is a problem since not both PC and mp3 player can access the players drive at the same time. If the player is connected to the PC via USB then the PC sees its filesystem and the player cannot access it anymore until disconnected.
2. Make the mp3 player act as a USB microphone. This means it needs to act as a sound card. This is a completely different use case for USB connection and requires the USB stack on the mp3 player doing something different. This is neither implemented right now nor is the Rockbox USB stack intended to do this (you need a different type of transfer messages for audio).
So while 2. should be possible it's not easy. And it's only possible for players that have USB via software.