As it so happens, there is an entire market for these types of devices. They're called audio interfaces, which are basically USB DACs (Digital Analog Converters) and most will be natively compatible with OS X, no drivers needed, since they use Core Audio. Then you have the side benefit of being able to connect instruments and microphones, since many of them have 48V phantom power, which is necessary for a lot of condenser microphones.Thanks guys.
Being more specific. I was using GarageBand out to a TC Helicon VoiceLive Play GTX via the headphone jack using a stereo to stereo cable (I have to speakers connected to the GTX's L&R Audio Out ports). I have a USB to stereo cable which I have connected from the MBP's USB to the 1/8" Aux In on the GTX, but no sound.
Has the MBP's headphone jack degraded over time, or was it always flaky?... Actually, I connect it to my iPad Air's headphone jack and the GTX works great. I was hoping to do the same with the laptop but it's headphone jack performs unacceptably. That's why I was hoping to use one of its USB ports instead.