The stimulus says nothing about how difficult it was for Beethoven to compose at any point in his life. We know hearing loss changed the quality of his music, but whether it made the process of composing harder or easier is a completely separate question the musicologist never addresses.

The musicologist says complete hearing loss gave Beethoven's later music a wonderfully introspective quality. But that doesn't mean his later music would have been worse overall without the hearing loss. Without it, his music might have developed other positive qualities that would have made it just as good or even better.

The musicologist only talks about a quality of Beethoven's music, not a quality of his personality. A composer can write highly introspective music without being an introspective person, just as someone can write cheerful music while being miserable inside. So we can't conclude anything about how introspective Beethoven himself would or wouldn't have been.

You might be thinking, "Beethoven's hearing loss was gradual, so maybe the introspective quality of his music also built gradually." But the musicologist specifically attributes the introspective quality to complete hearing loss, which arrived late in life. The fact that the process of going deaf was gradual doesn't mean each incremental step produced a little more introspection in the music. For all we know, the introspective quality arrived once hearing loss became total, not progressively over the decades from age 30 onward.

The musicologist tells us that complete hearing loss was what gave Beethoven's later music its introspective quality. If we take that seriously, then in a world where Beethoven never lost his hearing, his later music would probably have lacked that quality, meaning it would probably have sounded different.

On a Most Strongly Supported question, we don't need a logically airtight guarantee. We need to pick the best supported answer. When the stimulus tells us X caused Y, then we have good reason to believe that without X, Y probably wouldn't have happened either. That's not a Must Be True inference, because there could theoretically be other paths to Y. But it's enough for Most Strongly Supported.