Can you output to D+ and D- so that Arduino Uno can be used as a USB device? My project is basically a set of 5 audio signals (analog) that get altered slightly by Arduino Uno. I convert to MIDI and would then output it to USB, so that it can be used in a program of my choice on the computer. I found a vaguely similar project online with a guide. In that project they write a serial to a MIDI out, and then plug that into a computer. I'm fairly sure I can't just use the TX and run that into the USB, that wouldn't work. So I'm looking for an alternative method.
Not with an Uno, no. You can only use USB Serial (CDC/ACM) through the on-board USB chip[1].

Your best bet is probably with a Teensy 3 (or 3.1 or whatever they are up to now) which has built-in support for USB MIDI emulation.



1: I believe there is MIDI firmware for the USB chip on the Uno, but installing that requires a hardware programmer and then breaks the normal sketch upload.