The GO:PODCAST unit does not have an external input jack, so you cannot connect an external audio device or electric musical instrument to input its sound.
Alternatively, you can add audio files (background music, sound effects, etc.) from your smartphone to use as materials when streaming to the GO:PODCAST app. For details on importing files, please refer to "Adding still images, videos, and audio to a scene" in the GO:PODCAST app Owner's Manual.