Complex sentence: The initial ten episodes used a pronounced synthesizer sound (sometimes mistaken for a theremin), last heard on "The Corbomite Maneuver", while the remaining episodes of the season used an orchestral-only arrangement.
Split into simpler sentences: The initial ten episodes used a pronounced synthesizer sound (sometimes mistaken for a theremin), last heard on "The Corbomite Maneuver". The remaining episodes of the season used an orchestral-only arrangement.