。。
OK. With this, I can sing it by heart.
Literal
Yes. This [if-なら] by-heart can-sing [polite-ます].
そらで (空で) is the idiomatic phrase for '(doing something) from memory / by heart,' literally 'in the sky / in the air' — the metaphor being that the thing exists only in one's head with no written reference. なら 'if (given) this' hedges the claim as conditional on having this (probably a score or lyric sheet). 歌える is the potential form of 歌う 'to sing.'