So you are running one speaker, i.e. mono vs. Stereo (left and right speaker). You are either capturing only one side of the stereo field or you are collapsing both sides (left and right) into one speaker which is causing phase cancellation. Phase cancellation is an audio phenomenon that occurs when sound waves interfere with each other, eliminating certain frequencies. This can result in a dull or flat sound, and can make music sound weak, unclear, or even cause parts of the audio to disappear. This especially occurs in piano sounds that are sampled in stereo. Organs are mono instruments and samples are typically mono, so the same frequencies exist left and right,collapsing mono created samples have no impact where with stereo recorded instruments you get the phase cancellation.
Solution: two speakers, connect the left output to one and the right output to the other.