There are several ways to feed sound to the Enterprise Tape: input, but I don't recommend it to you unless you enjoy super-slow loads and squeaky chirps.....
As tofro suggest, HxC works perfectly on the Enterprise+EXDOS. Even there is a HxC Rom available to select disk images from the computer.
About SD-cartridges, no new batches since some years... The problem is, it is a private project from Szorg, and he hasn't freed the details so some other developer can build more units.