Two sound sources can be very easily mixed by 1)two condensers (use highest possible value),each connected to each sound source at one end (consider polarities!),the other ends tied together commonly connected to output or 2)one transformer.For efficiency and maximum energy transfer input and output impedances should be same.But in this case since a dc drive is used and inputs are 'squarish' not musical,without amplifier mixing will be possible but gain(loudness) may suffer.You can contact me at asrudra@vsnl.net if you need circuit details.