In the case of playing musical instruments, there is a specific idiom that is used: to play the [name of instrument]. When you "learn the [instrument]", you are really learning to play the [instrument]. Idioms don't usually make any logical sense, so it's possible that the usual rules for articles like 'a' and 'the' don't even apply in a very logical way in these examples.
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mitsuwao23I need to explainI've already given you the explanation. They sound unidiomatic.howwhat they sound like,