la refers to onde de choc
La colombe peut ressentir l’onde de choc.
In a metter of meaning, it refers to both, actually. You can’t separate “une sorte” and “onde“. La refers to “une sorte d’onde de choc“, it wouldn’t make sense to refer to “une sorte“.
But grammatically, it does refer to “onde“, as it would be “le” with a masculine noun.
La refers to onde de choc and agrees in gender with it.
Should a masculine substantive had be used, le would have been required, e.g.:
Ce truc génère une sorte de tremblement. La colombe peut le ressentir