Simplify the following text for a Grade 9 native English-speaking person.
Original: The quotations above show the importance of the metronome in the 20th century ( `` Most music teachers consider the metronome indispensable , and most professional musicians , in fact , continue to practice with a metronome throughout their careers '' ) .
Simplified: The `` MM '' stands for `` Maelzel 's Metronome '' . Beethoven was the first composer to use metronome markings , but he often gave very fast speed indications , and some people wonder whether he made mistakes or whether his metronome worked properly .