Looking at the UDOO schematic, the audio input jack is connected to MIC1 and MIC2 (Pins 21 and 22) of the Via VT1616 audio codec. Looking further at the Via VT1616 data sheet it can be seen that these go through a mux that selects one or the other before adding 20 dB gain and presenting it in MONO to the input selection mux. This is to allow the selection of either one of two microphones and _not_ a stereo channel. Put a stereo mic into the current jack and you'll be able to select either the left channel or the right channel but not a stereo channel. Stereo line-in is more appropriate for this device than is mono mic-in as it is currently designed. This simply must be revved to bring the audio input jack into LINE_IN_L and LINE_IN_R (pins 23 and 24) to be able to get a stereo signal into the board. Another input to MIC would be a good idea however.