Complex sentence: It was not the first time that doubts had been expressed over the authenticity of Concert Artist recordings, which also released recordings of the Chopin Mazurkas which, they claimed, had been recorded by Alfred Cortot.
Split into simpler sentences: It was not the first time that doubts had been expressed over the authenticity of Concert Artist recordings. The company also released recordings of the Chopin Mazurkas which, they claimed, were recorded by Alfred Cortot.