I have a behringer usb external soundcard. it works.
I have connected a Virus TI 1 with midi cables, and my music plays.
I have set the midi instruments to line in return.
I have tried just about every possible setting.
the synth does not render to WAV
Have you connected the audio out of the synth to a audio in on the soundcard?
Also make sure the Channel of the Line Input device is set to listen to the correct input on the soundcard.
As Jek said already. You need to insert the Line Input effect, and select the inputs you want to record. Be sure to render in real time, otherwise it won’t record the input.