so "datte" = what's more ?
You'd better not to think about the word だって singly. (Incidentally, だって is from だとて - "auxiliary verb だ + conjunctive particle と + case particle て".) HARISEN's line is a kind of complex sentence.
理由・判断の根拠を示す複文 - complex sentence which shows reason/source of judgment
「彼だって泳げるんだから、君だって泳げるよ」「彼だって泳げるだろう?だから君だって泳げるよ」: He can swim, so you'll be able to do as well.
In spoken Japanese, the latter sentence is quite often omitted.
「彼だって泳げるんだから」「彼だって泳げるだろ(う)?」
These mean "encouragement" (as far as it's said gently). "It's OK. You'll be able to swim as well. Don't be afraid..."
However, it's changed to the opposite meaning by adding 「が」 in the end of the sentence.
「彼だって泳げるだろうが!」(said angrily)
That's from 逆接の複文 - adversative complex sentence.
「彼だって泳げるだろうが!(なら君だって泳げるはずだ。なのになぜ泳げない!?)」
This means "reprobation". "He can swim. So you must be able to do. But why can't you!?"
In other expression, 「彼だって泳げるのに(なぜ君は泳げないの?)」
Back to HARISEN's, the sentence means「曲だってお前の好きなベートーヴェンだろうが!(だったらもっとうまく弾けるはずだ。なのになぜできない!?)」
literaly: The piece you played is Beethoven, your favorite! isn't it!?
as the implicature: Why can't you play better!?
多分もっと上手な言い回しの英訳があるでしょうね。
僕の英語力では思いつきませんが。
