Hi, when you say external USB soundcard do you mean a DAC? Did you install the firmware it came with (assuming it has firmware)?
One thing worth checking is the device manager in Windows, look under USB devices and see if the soundcard is recognized.
Another thing to try - go to Windows playback devices and disable everything apart from the soundcard, also disable Windows sound effects.
When you're running MusicBee, what playback mode are you using, DirectSound, WASAPI, etc.? Whatever the case, try each of them to see if it helps.