I think it can be something like "Let's pile up breaths" (whatever that means).
Iki can be translated as "breath" or "tone" (plurality is not indicated in Japanese).
Kasanemashou is the polite volitional form of Kasaneru. Kasaneru can be translated as "to pile up", or "to put something on another", or "to heap up", or "to add", or "to repeat".