Since R-07 OS version 1.10 you can connect the R-07 via USB to your computer, iPhone, or iPad, and use it as a USB audio interface.
USB Level sets the volume of a signal that comes from computer/iOS gadget to your R-07 phones jack as shown below:
Details are covered on page 4 of the manual "Functions Added in R-07 System Program Ver.1.10".
You can also download this manual from the link below:
https://www.roland.com/global/products/r-07/support/
* When using the R-07 as a USB audio interface, files inside the R-07 cannot be played back. Nor is it possible to record into the R-07.