First thing you should do is ensure you are using the current and proper AudioBox device drivers.
24 bit audio can always go down to 16. Conflicts here are program specific. You may need to contact Cakewalk.
Otherwise, check the Playback Devices. Check the Properties of the AudioBox and click the Advanced tab. Here you see a Default Format. Set this to 44.1k Sampling Rate and 16 bits. Try again.
Of course, there are also many other MP3 converting apps out there that may not have this limitation.