You shouldn't regard "topic" or "contrast" as
a priori something accepted from the beginning. They are just grammar theories. Different people can explain は's function either with solely topic or with solely contrast.
Some call it "contrast" when you can find implication that something is different from something else, and others call secondary topics "contrastive は" and primary topics "topic".
What's sure about は is that it highlights the thing marked with it and separates it and lets it float around the rest of the clause.
As for your example, you can interpret it as "As for dog's bark, it's audible. But as for its figure, it's not visible".
Speaking of emphasis, you can put it on either 犬の姿は or 犬の姿が by stress in pronunciation. (If you emphasize contrast between voice and figure, it's better to omit 犬の from 犬の姿.)
If the sentence was 声は…犬の姿が見えない (without が stressed*1), the latter sentence would be a sentence of neutral description in this case and the fact that you can't see them feels abrupt and your mind feels kind of renewed and occupied with it. In contrast, the original sentence feels like taking 犬の姿 as only one of agenda (without は stressed).
*1: In that case, it's interpreted as exhaustive listing が.