Welcome to RO.
Use the tape outs on the Behringer so you don't have to convert anything yourself.
Try the USB interface or connect the tapeouts directly to the RCA inputs of your Audigy. Decide what sounds best. Just make sure your cabling isn't too cheap.
I wouldn't use the 1/4 (mic 2) input. It doesn't sound like an input for line signals. I suspect it will cause impedance issues which degrades your signal, add noise and introduce distortion. I think it's only meant for typical PC microphones.
Line signals: there's -10dB and +4dB. The tapeouts on your Behringer are unbalanced -10dB and the XLRs are balanced +4dB. If the Audigy RCA inputs can handle a cassettedeck or CD-player then they should be able to handle the RCA outputs of the Behringer.
Are we talking about an electric bass guitar here? Does it go straight into the board? Active or passive electronics? It might help if the bass is a little preprocessed before it enters the board with at least a DI, preferably a bass preamp. If slapping is involved you need some limiting too.