Revealing mistake: Towards the end when the Phantom's lair is collapsing, you can see bricks falling into the sewer water and floating. Obviously they were made out of light-weight material and not out of actual stone.
Continuity mistake: The Phantom's entire face is hit by the tray of acid, yet only the right side of his face gets disfigured.
Continuity mistake: The Phantom's right eye only becomes disfigured once he's unmasked - it's normal the rest of the time.
Plot hole: The opera house is directly above the Phantom's hideout in the sewers, with an orchestra booming loudly overhead almost every night (the Phantom plainly hears the orchestra), but this constant vibration doesn't affect the integrity of the subterranean structure. But at the end, the noise of two simple gunshots causes the Phantom's hideout and a massive part of the sewer system to collapse.
Answer: No, Claude Rains was not a trained musician. He was coached on how to play the violin and piano, but Rains' playing was dubbed over in post-production.
raywest ★