When you are overdubbing, it is important that the track you are playing and are recording are at the same clock rate; otherwise you are going to have problems. Double-check Audacity’s sample rate in the lower left-hand corner with that of Windows: R-click: [speaker icon] > Sounds > Recording > [Behringer UM2] > Properties > Advanced > Default Format. Repeat this for your playback device. These must match your UM2. (I believe Behringer is usually 48000Hz).
If you wan to go back to ASIO. See here: ASIO support in Audacity 2.4.2 - #13 by jademan
I hope this helps. 